The correct procedure is to copy the partition table from the surviving disk to the replacement using sfdisk, then add the new partition to the array with mdadm --manage --add. This is required because mdadm expects the new disk to have a matching partition layout, including the Linux RAID autodetect type (0xFD), so that the RAID superblock can be properly recognized and synchronized. On the Red Hat Certified System Administrator EX200 exam, this task tests your understanding that mdadm manages partitions, not raw disks, and a common trap is attempting to add the raw disk directly without first replicating the partition table. Remember the sequence: partition table first, then add to array. A useful memory tip is "sfdisk before mdadm" — think of it as "copy the map before joining the team."
EX200 Create and configure file systems Practice Question
This EX200 practice question tests your understanding of create and configure file systems. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A system has two 500GB disks in a RAID1 (mirror) using mdadm. One disk fails. After replacement, what is the correct procedure to restore redundancy?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use sfdisk to copy partition table from /dev/sda to /dev/sdb, then 'mdadm --manage /dev/md0 --add /dev/sdb1'
Option D is correct because when replacing a failed disk in a RAID1 array managed by mdadm, the new disk must have a partition table that matches the surviving disk. Using sfdisk to copy the partition table from /dev/sda to /dev/sdb ensures that the partition layout (e.g., partition type 0xFD for Linux RAID autodetect) is identical, which is required before adding the partition (e.g., /dev/sdb1) to the array with mdadm --manage --add. Simply adding the raw disk without a proper partition table would fail because mdadm expects a partition with the correct RAID superblock and partition type.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
Run 'mdadm --manage /dev/md0 --add /dev/sdb'
Why it's wrong here
Need to add partition, not whole disk.
✗
Remove the failed disk with 'mdadm /dev/md0 --fail /dev/sdb1' then add new.
Why it's wrong here
If disk is already failed, removal may not be needed.
✗
Run 'mdadm --assemble --scan' to rebuild the array automatically.
Why it's wrong here
Assemble only for existing arrays.
✓
Use sfdisk to copy partition table from /dev/sda to /dev/sdb, then 'mdadm --manage /dev/md0 --add /dev/sdb1'
Why this is correct
Correct steps.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume adding a new disk directly with 'mdadm --add' is sufficient, overlooking the critical prerequisite of having an identical partition table on the replacement disk, which is a common oversight in RAID recovery procedures.
Detailed technical explanation
How to think about this question
Under the hood, mdadm stores RAID superblock metadata at the end of each partition (or at a specific offset) to track array membership and state. When a new disk is added, mdadm expects the partition to have the correct partition type (0xFD for Linux RAID autodetect) and a clean superblock area; copying the partition table with sfdisk ensures the partition boundaries and type match the surviving disk, preventing geometry mismatches. In real-world scenarios, if the new disk is from a different vendor or has slightly different geometry, sfdisk -d /dev/sda | sfdisk /dev/sdb can fail if the disk sizes differ, requiring manual partition alignment or using 'mdadm --add' with the whole disk only if the array was originally created on whole disks without partitions.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the EX200 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Create and configure file systems — This question tests Create and configure file systems — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use sfdisk to copy partition table from /dev/sda to /dev/sdb, then 'mdadm --manage /dev/md0 --add /dev/sdb1' — Option D is correct because when replacing a failed disk in a RAID1 array managed by mdadm, the new disk must have a partition table that matches the surviving disk. Using sfdisk to copy the partition table from /dev/sda to /dev/sdb ensures that the partition layout (e.g., partition type 0xFD for Linux RAID autodetect) is identical, which is required before adding the partition (e.g., /dev/sdb1) to the array with mdadm --manage --add. Simply adding the raw disk without a proper partition table would fail because mdadm expects a partition with the correct RAID superblock and partition type.
What should I do if I get this EX200 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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