# Data remanence

> Source: Courseiva IT Certification Glossary — https://courseiva.com/glossary/data-remanence

## Quick definition

When you delete a file or format a drive, the data isn't always completely gone. Tiny traces of the original information can still be recovered using special tools. This leftover data is called data remanence, and it's a big security risk if sensitive information ends up in the wrong hands.

## Simple meaning

Imagine you write a note in pencil on a piece of paper, then you erase it with a rubber. You can still see the indentations left by the pencil, and if you rub the paper with the side of another pencil, the old writing shows up again. That's a bit like data remanence. On a computer, when you delete a file, the computer doesn't actually scrub the magnetic particles or electronic charges that made up that file. Instead, it just marks the space as available for new data. Until that space is overwritten by something else, the original data is still there and can be recovered. 

Think of a whiteboard. You write something, then you wipe it off. From a distance it looks clean, but up close you can still see faint traces of the marker. If someone sprays water on the board, those traces become more visible. The same happens with hard drives, solid-state drives, and even memory sticks. The physical medium retains a ghost of the data. This matters because if you sell, donate, or throw away a device, someone with the right tools can pull up your deleted files, passwords, photos, or business secrets. 

For IT professionals, especially those studying for security certifications, understanding data remanence is crucial. It tells us that simply hitting the delete key or performing a quick format is not enough to protect sensitive information. We need special methods like overwriting with random data, degaussing (using strong magnets), or physically destroying the media to make sure the data is truly gone. It's a core concept in risk and asset security because it deals with how we protect data at the end of its life.

## Technical definition

Data remanence refers to the measurable physical or logical remnants of data that persist on a storage medium after standard data removal attempts. In IT security, this is a serious concern because it can lead to unauthorized disclosure of sensitive information. The phenomenon occurs due to the way different storage technologies encode and retain data. On magnetic media like Hard Disk Drives (HDDs), data is written by magnetizing tiny regions on the platters. A standard delete operation only removes the file system pointer, leaving the magnetized patterns largely untouched. Even after a full format, the magnetic domains may still retain a residual magnetic flux that can be read using specialized equipment like magnetic force microscopes or by analyzing the analog signal from the read head. 

On solid-state drives (SSDs) and other NAND flash memory, data remanence manifests differently. SSDs use floating-gate transistors where charge is trapped in an oxide layer to represent bits. Standard erase commands (such as ATA TRIM) mark blocks for garbage collection but do not physically remove all trapped charge. Over time and under certain conditions, the threshold voltage of these floating gates can drift, and with careful measurement, the original state of a cell can be inferred even after erasure. This is known as charge-based remanence. SSDs have over-provisioning and wear-leveling algorithms that move data around, sometimes leaving copies in unexpected physical locations that are not exposed to the operating system's erase commands. 

Other volatile memory types like Dynamic Random-Access Memory (DRAM) and Static Random-Access Memory (SRAM) also exhibit remanence. In DRAM, data can persist for seconds or even minutes after power is removed due to residual charge in the capacitors, especially at low temperatures. Cold boot attacks exploit this to recover encryption keys from memory. Standards such as NIST SP 800-88 (Guidelines for Media Sanitization) define three levels of data removal: Clear (overwrite with a simple pattern), Purge (degaussing or block-level erase for SSDs), and Destroy (physical shredding or incineration). For high-security environments, cryptographic erasure (encrypting data and then destroying the key) is also considered an effective method, because even if the encrypted data remains, it is unreadable without the key. 

In real IT implementation, data remanence affects asset disposal policies, data center decommissioning, and incident response. Enterprises must document sanitization methods for each type of media. For example, an organization may specify that all HDDs be degaussed and then shredded before disposal, while SSDs are subjected to a secure erase command followed by a cryptographic erase. Failure to address data remanence can lead to data breaches, regulatory non-compliance (GDPR, HIPAA), and legal liability. The (ISC)² CISSP exam covers this topic under Domain 2: Asset Security, focusing on the proper disposal and re-use of media.

## Real-life example

Think about writing a letter with a ballpoint pen on a pad of paper. You finish the letter, decide you don't want it, so you tear off that page and throw it in the trash. To you, the letter is gone. But someone who finds that torn-out page can read everything you wrote. Now, imagine you decide to be more careful. You take the page and scribble all over it until the original writing is hidden. You still threw the paper away, but now the original message is much harder to read. That's closer to what a basic overwrite does. 

Now, consider a more advanced scenario. You have a legal notepad. You write your letter, then you carefully rub the paper with sandpaper until the surface is worn down and the ink is gone. You've physically removed the top layer. This is like degaussing a hard drive, using a powerful magnet to disrupt the magnetic fields that store the data. Even then, microscopic fragments of ink might remain in the fibers of the paper. With a magnifying glass, someone could piece together bits of the original letter. That is the reality of data remanence. 

If you really want that letter to be unreadable forever, you would burn the paper in an incinerator and then scatter the ashes. That's physical destruction, the only way to guarantee that data cannot be recovered, just like shredding a hard drive into small pieces. This everyday analogy helps IT learners understand why data remanence is a security risk: simply deleting or formatting is like throwing away the whole pad without tearing out the page. The data is still there, just out of sight. Proper disposal methods must match the sensitivity of the data. Understanding that data leaves physical traces, even after we think it's gone, is the foundation of media sanitization.

## Why it matters

Data remanence matters because it directly impacts the confidentiality of information throughout the entire lifecycle of a storage device. In practical IT, organizations handle huge volumes of sensitive data, customer records, financial statements, intellectual property, employee information, and encryption keys. When a device reaches its end of life, whether through upgrade, failure, or simple replacement, the data stored on it must be rendered unrecoverable. If an organization neglects proper sanitization, they could inadvertently expose that data to the next user of the device, a buyer at an electronics recycler, or a malicious actor who finds the device in a dumpster. 

Data breaches caused by improper disposal are not just hypothetical. There have been real-world incidents where hard drives sold on secondary markets still contained medical records, credit card numbers, or classified government documents. The cost of such breaches includes financial penalties, legal fees, reputation damage, and loss of customer trust. For IT professionals, knowing how to assess the risk of data remanence and apply appropriate controls is a core job responsibility. This includes selecting the right sanitization method based on the data classification, the type of media, and the security requirements of the organization. 

Another practical context is in cloud computing and virtualized environments. When a cloud tenant deletes a virtual machine or releases storage volumes, the underlying physical storage may be reallocated to another tenant. Without proper sanitization (like zeroing out blocks), data remanence could allow one tenant to read another tenant's old data. Hypervisors and cloud platforms use secure deletion features, but IT professionals need to verify that these mechanisms are properly configured. In incident response, understanding data remanence is also critical, analysts may need to recover evidence from partially overwritten drives, or they must ensure that compromised systems are sanitized before repurposing. 

Finally, data remanence affects compliance with legal and regulatory frameworks. Laws like GDPR require that personal data be erased when it is no longer needed. A simple delete is not sufficient under these regulations; organizations must demonstrate that data has been irrecoverably removed. Similarly, standards like PCI DSS for credit card data mandate specific sanitization procedures for storage media. Ignoring data remanence can lead to non-compliance and heavy fines. For these reasons, data remanence is not just a theoretical concept but a fundamental aspect of information security governance.

## Why it matters in exams

Data remanence is a core topic in the (ISC)² CISSP exam, specifically under Domain 2: Asset Security. It is also part of the broader risk management and security operations domains. In CISSP, candidates are expected to understand the different states of data (at rest, in transit, in use) and how to protect each state. Data remanence directly relates to data at rest, particularly when that data is on media that is being decommissioned or reused. The exam objectives explicitly mention media sanitization techniques and when each is appropriate. You might be asked to compare and contrast clearing, purging, and destruction, or to recommend the correct method for a given scenario (e.g., an SSD vs. an HDD, or a device with classified vs. unclassified data). 

Other exams, such as CompTIA Security+, also cover data remanence in the context of data destruction and disposal procedures. The Security+ exam objectives include topics like secure disposal, recycling, and data remanence. You may see questions about the difference between wiping, degaussing, and shredding. The Certified Information Security Manager (CISM) exam also touches on this when discussing asset management and data leakage prevention. However, for the purpose of this glossary, the primary exam relevance is the CISSP. 

In CISSP questions, data remanence often appears as part of a scenario where a company is upgrading its servers and needs to dispose of old hard drives. The question will present a situation with varying security requirements (e.g., public-facing data vs. trade secrets) and ask which sanitization method is most appropriate. Alternatively, you may be asked about the residual risk associated with a particular method. For example, a question might describe an organization that simply formats drives before donating them, and the answer would point out the risk of data remanence. Another common question type is about the order of operations: what to do with media that is leaving the organization's control. 

Exam questions also test your understanding of specific standards. You might be asked which NIST guideline addresses media sanitization (NIST SP 800-88). Or you could be given a list of methods and asked to identify which one leaves the highest residual risk. The traps often involve confusing clearing with purging or assuming that a quick format is sufficient. Another trick is mixing up the effectiveness of degaussing for different media types (e.g., degaussing works on HDDs but not on SSDs). 

For the CISSP, you should also know about cryptographic erase as a modern alternative. This involves encrypting the entire drive and then destroying the encryption key, making the remaining encrypted data unrecoverable. This method is gaining popularity in cloud environments. The exam might test this as the preferred method for SSDs because it avoids the wear and performance impact of multiple overwrites. Overall, candidates need to be precise about definitions, methods, and their appropriate applications. Understanding data remanence helps you answer scenario-based questions and is a high-frequency topic in the asset security domain.

## How it appears in exam questions

In certification exams, data remanence questions typically fall into three patterns: scenario-based, method comparison, and troubleshooting. Scenario-based questions present a business situation and ask for the best sanitization approach. For example: A financial institution is decommissioning 100 hard drives containing five years of customer transaction records. The drives are still functional and the company plans to sell them to a refurbisher. Which of the following should be performed FIRST? The options might include quick format, full format with overwrite, degaussing, or drive shredding. The correct answer is often degaussing (purge level) because it makes the data unrecoverable while allowing the drive to be reused (though the refurbisher would need to know the drive cannot be reused after degaussing, careful with that nuance). Alternatively, if the drives contained top-secret data, the answer might be physical destruction. 

Method comparison questions directly ask you to differentiate techniques. For instance: Which of the following sanitization methods leaves the data on the media but prevents the operating system from accessing it? The answer is clearing, because it overwrites only the areas accessible to the OS, leaving potential residual data in bad sectors or service areas. Or: Which method is most effective for removing data from an SSD? The correct answer is cryptographic erase, because SSDs have over-provisioning and wear leveling that make multiple overwrites unreliable. Questions may also ask about the level of data remanence after each method, which leaves the least residual data? 

Troubleshooting-style questions might present a scenario where a company performed data sanitization but a subsequent security audit found recoverable data. You would need to identify the reason, such as failure to account for hidden areas like the host protected area (HPA) or device configuration overlay (DCO) on HDDs, or failure to use a secure erase command on SSDs. Another troubleshooting pattern: A user complains that after wiping a drive, the operating system still shows the old partition table. This indicates the wipe was done at the file level but not the full disk level. 

There are also questions about the effect of wear leveling on SSDs. An exam scenario might say: A security administrator performed a full overwrite using a tool that writes zeros to the entire logical block address space. However, forensic analysis still recovered old data. Why? Because the SSD's controller did not actually overwrite all physical blocks due to over-provisioning. The correct fix would be to use an ATA Secure Erase command or encrypt the drive before decommissioning. 

Finally, some questions test knowledge of residual data in volatile memory. For example: After powering off a server, why might encryption keys remain recoverable? Because of DRAM remanence, especially if the system is quickly rebooted or cooled below -20°C. This is a classic cold boot attack scenario. Such questions are less common but appear in advanced security exams. By understanding how data remanence appears in questions, learners can focus on the practical decision-making aspects rather than just memorizing definitions.

## Example scenario

You work as a junior IT administrator for a mid-sized company. Your boss hands you a box of 20 used external USB hard drives that were previously used by the finance department to store monthly backups for the past three years. The department recently moved to a cloud backup solution, so these drives are no longer needed. Your boss tells you to “erase them and give them to the local school for their computer lab.” He mentions that the drives contain financial reports from 2020 to 2023, but says the data is old and no longer sensitive. You know better. You decide to handle this properly. 

First, you understand that simply deleting files or performing a quick format will leave data remanence on the drives. Even though the finance department says the data is not sensitive, it still contains internal financial information that could be used for competitive intelligence or targeted phishing attacks. You decide to use a dedicated disk wiping tool that performs a multi-pass overwrite with random data, following the NIST SP 800-88 Clear standard. However, you also recall from your CISSP studies that external hard drives may have bad sectors where the overwrite tool cannot write. Those sectors can still hold remnant data. To address this, you run a diagnostic that marks bad sectors, but the wiping tool you have does not write to them. You then decide that a better approach is to use a full ATA Secure Erase command if the drives support it, because that command is issued at the firmware level and can access all sectors. 

Even after secure erasure, there is still a theoretical risk of data remanence using specialized laboratory techniques. For drives going to a school, this risk is low but present. You document the sanitization method used for each drive, including the serial number and date. You then test a couple of the erased drives with a file recovery tool to confirm that no readable data can be recovered. Only then do you hand them over. 

This scenario illustrates the practical application of data remanence knowledge. It shows that IT professionals cannot rely on simple deletes. They must select the correct sanitization method based on the residual risk, the sensitivity of the data, the intended future use of the media, and the technical capabilities of the drives. It also highlights the importance of documentation and verification. In an exam, a similar scenario would ask you to pick the best course of action from a list of options, often with the correct answer being a combination of secure erase and verification.

## Common mistakes

- **Mistake:** Believing that deleting a file and emptying the recycle bin permanently removes the data.
  - Why it is wrong: Deleting a file only removes the file system reference to the data. The actual bits remain on the storage media until they are overwritten. This is the textbook definition of data remanence. Forensic tools can easily recover these files.
  - Fix: Always use a dedicated wipe tool that overwrites the storage space with zeros or random patterns at least once (for most scenarios). For high security, use degaussing or physical destruction.
- **Mistake:** Assuming that a quick format is sufficient to remove all data from a hard drive.
  - Why it is wrong: A quick format only clears the file system metadata, not the data in the sectors themselves. The original files remain recoverable. A full format on modern operating systems may perform a simple pass but often does not cover all areas like the Host Protected Area (HPA) or Device Configuration Overlay (DCO).
  - Fix: Perform a full overwrite of the entire logical block address space, and if available, use a manufacturer-provided secure erase command to access all physical sectors, including hidden areas.
- **Mistake:** Thinking that degaussing works on SSDs the same way it works on HDDs.
  - Why it is wrong: Degaussing uses a strong magnetic field to disrupt the magnetic domains on an HDD platter. SSDs store data in electrical charges within NAND flash cells, not magnetic patterns. A magnetic field will not erase an SSD. In fact, degaussing an SSD can physically damage the controller but leave the flash memory chips intact and readable.
  - Fix: Use cryptographic erasure or an ATA Secure Erase command for SSDs. For physical destruction, shredding or incineration is effective.
- **Mistake:** Assuming that overwriting an SSD once with zeros is enough to prevent data recovery.
  - Why it is wrong: SSDs have wear-leveling and over-provisioning. The operating system's overwrite command only hits logical block addresses, but the SSD controller may map those addresses to different physical blocks over time. Some old data may remain in unallocated physical blocks that are not accessible by the OS write command. This is a common exam trap.
  - Fix: Use the ATA Secure Erase command, which is issued at the drive firmware level and forces the SSD to erase all blocks, including the over-provisioned area. Alternatively, enable full-disk encryption before the drive is used and then destroy the encryption key.
- **Mistake:** Believing that physically smashing a hard drive with a hammer is a reliable destruction method.
  - Why it is wrong: Hammering may damage the casing and platters, but forensic techniques can still reassemble fragments and read data from intact platter surfaces. Partial platter pieces can yield significant amounts of data. A hammer does not guarantee complete data destruction.
  - Fix: Use an industrial shredder designed for hard drives, or incinerate the drives at high temperatures. For sensitive data, combine degaussing with shredding.

## Exam trap

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## Commonly confused with

- **Data remanence vs Data at rest:** Data at rest refers to any data stored on a medium (hard drive, SSD, tape) that is not currently being accessed or transmitted. Data remanence is a specific property of that data, the persistence of its remnants after deletion. Think of data at rest as the category, and data remanence as the side effect of deletion. (Example: A spreadsheet stored on a server is 'data at rest.' When you delete that spreadsheet, the bits that still remain on the drive are 'data remanence.')
- **Data remanence vs Data degradation:** Data degradation (bit rot) is the gradual loss of data integrity due to physical media decay over time, such as magnetic domain weakening in HDDs or charge leakage in SSDs. Data remanence is about data persistence after deletion, not about natural decay. They are opposite in nature, one is loss, the other is unwanted survival. (Example: An old CD-R that becomes unreadable after 10 years is data degradation. A file you deleted from that CD-R yesterday that can still be recovered is data remanence.)
- **Data remanence vs Secure deletion:** Secure deletion is the process of actively eliminating data remanence. Data remanence is the problem; secure deletion is the solution. They are closely related but not the same. You cannot talk about one without the other in security contexts. (Example: Using a tool like 'dd' to overwrite a drive with zeros is an act of secure deletion. The leftover magnetic signature that might remain even after that is data remanence.)
- **Data remanence vs Memory leak:** A memory leak is a software bug where a program fails to release memory that it no longer needs, causing the memory to remain allocated and potentially causing the system to run out of memory. Data remanence is about physical data persistence, not about software resource management. (Example: A memory leak might cause a browser to slow down over time. Data remanence might allow an attacker to read your old password from an erased hard drive.)

## Step-by-step breakdown

1. **File Deletion** — When a user deletes a file, the operating system removes the file system entry (the pointer to where the data is stored). The actual data bits on the storage medium remain intact. At this point, data remanence is maximum, the data is still fully recoverable with simple tools.
2. **Quick Format** — A quick format removes the file system structure (e.g., the master file table on NTFS). It marks all space as available, but does not write new data over the existing bits. Data remanence remains high. The data can still be recovered with file forensics software.
3. **Full Format (Overwrite)** — A full format writes zeros or a specific pattern to the entire logical disk area. This overwrites most user-accessible data. However, areas like bad sectors, Host Protected Area (HPA), and Device Configuration Overlay (DCO) may not be overwritten. Data remanence is reduced but not eliminated. In SSDs, logical overwrite may not reach all physical blocks due to wear leveling.
4. **Purge (Degaussing or Secure Erase)** — Degaussing exposes the drive to a strong magnetic field that randomizes the magnetic domains on the platters, making data unrecoverable via standard means. Secure Erase (for ATA drives) sends a firmware-level command that forces the drive to erase all physical blocks, including over-provisioned areas. This greatly reduces data remanence. For HDDs, degaussing destroys the drive's ability to function. For SSDs, Secure Erase is the preferred purge method.
5. **Cryptographic Erase** — If the drive was encrypted with a strong algorithm (e.g., AES-256), the data on the drive is scrambled. To sanitize, you simply destroy the encryption key. Without the key, the encrypted data is computationally infeasible to decrypt, effectively neutralizing data remanence. This is often used for SSDs and cloud storage because it is fast and avoids physical wear.
6. **Physical Destruction** — This involves physically breaking the storage medium into small pieces. Methods include shredding (industrial equipment that pulverizes drives), crushing, incineration, or dissolving in acid. Physical destruction is the only method that guarantees data cannot be recovered, even with advanced laboratory techniques. It is used for the highest security classifications (e.g., Top Secret).

## Practical mini-lesson

Data remanence is not just a theoretical concept; it is a daily operational concern for IT professionals. When you are responsible for the lifecycle of storage devices, you must decide on sanitization methods based on the classification of the data, the type of media, and the acceptable residual risk. Let's walk through a typical scenario to see how this plays out in practice. 

You are the security lead for a company that is migrating from physical servers to a cloud infrastructure. You have a rack of 48 servers, each with two 2TB HDDs and one 512GB SSD (used for caching). The hardware will be returned to the leasing company. According to the lease agreement, the data must be completely removed. The leasing company will resell the servers to a secondary market. What do you do? 

First, you separate the drives by type. For the HDDs, you decide on a two-step approach: degaussing followed by a full overwrite of zeros. But wait, degaussing will make the drives non-functional, which violates the lease requirement to return working drives. You check the contract: it says “data removed, drives functional.” So degaussing is out. You then choose a multi-pass overwrite using a tool like DBAN (Darik's Boot and Nuke) that supports the NIST 800-88 Clear method. However, DBAN does not guarantee that the HPA or DCO areas are overwritten. You decide to use a drive diagnostic tool (like HDDerase) that can issue a Secure Erase command, which is a purging method. Secure Erase will make the drives functional again after the command completes, and it accesses all physical sectors. For the SSDs, you cannot use Secure Erase alone because of the potential for residual charge? Actually, ATA Secure Erase for SSDs is effective if the drive firmware implements it correctly (most modern SSDs do). You perform a Secure Erase on each SSD, and then also ensure the drives are encrypted at the firmware level (self-encrypting drives) and you wipe the encryption key (but since the drive is being returned, you would not have a key to keep). You document the serial numbers, the date, and the tool used for each drive. You then test a sample of drives by attempting to recover data with a forensic tool. 

What could go wrong? If you use a bootable wiping tool that runs from a CD, it might not support the specific hardware RAID controller. In that case, the drives appear as a single logical volume, and you need to break the RAID before wiping. Another issue: some SSDs may have a power loss during the Secure Erase command, leaving the drive in an indeterminate state. Always have a backup plan. 

Professionals also need to know about cryptographic erase in environments using full-disk encryption (like BitLocker or FileVault). If the drives are encrypted, simply deleting the encryption key is often faster and more secure than overwriting. The data remains on the drive, but it is unreadable. However, be aware that some older encryption implementations may leave plaintext remnants in the unused space of the file system. For true elimination, encrypt the entire drive before any data is written, and then destroy the key at decommissioning. This lesson teaches that data remanence management is a process that requires careful planning, knowledge of the hardware, and verification. It is not a one-size-fits-all task.

## Memory tip

Data Remanence: D.R. = Data Remains. Think: 'Delete is not dead.' Deleted data is still there until overwritten.

## FAQ

**Is data remanence only a problem with hard drives?**

No, data remanence occurs on all types of storage media: hard drives, SSDs, USB flash drives, memory cards, RAM, tapes, and even paper (e.g., carbon paper). Any medium that can store data has some form of remanence.

**Can data be recovered after a factory reset on a phone?**

A factory reset on a phone typically performs a logical wipe, not a full overwrite. Data remanence on the internal flash memory may allow recovery using forensic tools, especially if the reset does not trigger a secure erase command on the storage chip.

**What is the difference between clearing and purging?**

Clearing (overwriting) is a logical sanitization that prevents data recovery using standard file system methods but may leave residual data that can be recovered with advanced techniques. Purging (degaussing or secure erase) makes data unrecoverable even with special tools and is more thorough.

**Does formatting a hard drive remove data remanence?**

A quick format does not. A full format that overwrites all sectors does remove most data, but it may not reach hidden areas like the Host Protected Area. For complete removal, use a dedicated sanitization tool.

**Why is data remanence a bigger issue with SSDs than HDDs?**

SSDs have over-provisioning and wear-leveling, meaning data may be copied to multiple physical locations over time. Even after a logical overwrite, old data can remain in unused blocks. The NAND flash cells can retain residual charge, making data recovery possible with specialized equipment.

**What is the best method for sanitizing an SSD?**

The best method is to use an ATA Secure Erase command, which is a firmware-level operation that erases all physical blocks. Alternatively, if the SSD supports hardware encryption, cryptographic erase (destroying the encryption key) is also effective.

## Summary

Data remanence is the property of stored data that persists even after deletion or sanitization attempts. For IT security professionals, understanding this concept is essential because it directly affects the confidentiality of data at the end of its lifecycle. Simply pressing delete or performing a quick format leaves data recoverable, posing a significant risk if storage media leaves an organization's control. 

We covered the difference between clearing, purging, and destroying, and how each method addresses remanence to varying degrees. Real-world practice involves selecting the right method based on data sensitivity, media type, and compliance requirements. For HDDs, degaussing and physical destruction are strong options. For SSDs, cryptographic erase and Secure Erase are preferred. We also saw how data remanence appears in certification exams, particularly in the CISSP Asset Security domain, where scenario-based questions test your ability to choose the appropriate sanitization method. 

Key takeaways: never rely on deletion alone; always verify that the sanitization method matches the media; and remember that the only way to guarantee zero data remanence is physical destruction. For most organizations, a combination of overwriting and secure erasure is sufficient. As a final exam tip, focus on the precise definitions of clearing, purging, and destruction, and practice applying them to different storage technologies. Understanding data remanence is not just about passing an exam, it's about ensuring that your organization's data stays protected even after its useful life has ended.

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Practice questions and the full interactive page: https://courseiva.com/glossary/data-remanence