CHFI OS and File System Forensics Practice Question
During a forensic investigation, an analyst needs to preserve the integrity of evidence on a hard drive. Which of the following is the best practice for acquiring an image of the drive?
⚠ Common exam trap
The CHFI exam often tests the misconception that a software-based approach (like dd) is sufficient for imaging, but the trap is that without a hardware write blocker, the forensic workstation's OS may inadvertently write to the suspect drive (e.g., via automount or journaling), compromising evidence integrity.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use a hardware write blocker and create a bit-stream image.
Using a hardware write blocker ensures that no write commands from the forensic workstation reach the suspect drive, preserving its integrity at the physical level. Creating a bit-stream image (sector-by-sector copy) captures all data, including slack space and unallocated clusters, which is essential for thorough forensic analysis. This combination is the gold standard in digital forensics, as mandated by best practices like those from NIST and the ACPO principles.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Use the 'dd' command to create a raw image without a write blocker.
Why it's wrong here
The 'dd' command performs a bit-for-bit raw acquisition, but without a hardware write blocker it leaves the original drive exposed to the operating system's automatic writes, such as mounting, journaling, or updating access timestamps. Because 'dd' runs at the OS level, it cannot intercept or block the underlying ATA/SCSI write commands that a genuine write blocker would prevent, so even a well-intentioned 'dd' invocation risks altering the evidence and breaking the chain of custody.
- ✗
Connect the drive to a forensic workstation and use the operating system's copy command.
Why it's wrong here
Using the operating system's copy command, such as dragging and dropping files, requires the OS to mount the drive, which typically engages the file system with read-write access and can modify access times, update file system journal logs, or write temporary metadata. Furthermore, a logical copy only captures active files and does not include deleted files, unallocated space, slack space, or alternate data streams, all of which are critical to forensic analysis; the copy also fails to preserve the exact byte-for-byte state of the original media.
- ✓
Use a hardware write blocker and create a bit-stream image.
Why this is correct
A hardware write blocker is the definitive forensic safeguard: it sits between the drive and the workstation and physically intercepts any write command at the ATA/SATA/USB interface, allowing only read operations. Creating a bit-stream image (e.g., using 'dd' or FTK Imager) then captures a sector-by-sector, bit-identical copy of the entire drive, including partition tables, unused sectors, slack space, and deleted files, ensuring the original evidence remains pristine and admissible in court.
- ✗
Format the drive before imaging to ensure no hidden data is missed.
Why it's wrong here
Formatting the drive is catastrophic in digital forensics because it initializes a new file system, overwriting the master boot record, file allocation tables, and inode structures, and in many cases also erases directory entries and data clusters, destroying active files and much of the residual data. This process does not expose hidden data—it actively erases it, and any 'hidden' data that survives is only in unallocated space, but formatting makes recovery far less likely. Forensic acquisition must preserve the original media untouched; formatting violates every principle of evidence integrity and is never acceptable.
Go deeper
Related to this question
Learn chapter
Overview of Computer Forensics and Investigation Process
Key term
Evidence Admissibility
Evidence admissibility is the legal and technical standard that determines whether digital evidence can be used in a court of law.
Key term
FTK Imager
FTK Imager is a free forensic imaging tool used to create exact copies of computer drives and storage devices for digital evidence analysis.
About these practice questions
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This CHFI practice question is part of Courseiva's free EC-Council certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the CHFI exam.