This EX200 practice question tests your understanding of configure local storage. 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.
An administrator replaces a failed disk in a RAID 10 array /dev/md0. The new disk is /dev/sdc. The admin runs: mdadm /dev/md0 --add /dev/sdc. The command succeeds, but the array does not start rebuilding. What is the most likely reason?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The failed disk was not removed from the array first; you need to mark it as failed and remove it.
Option D is correct because when a disk in a RAID array fails, the array marks it as faulty but does not automatically remove it. The administrator must first explicitly mark the failed disk as failed with `mdadm --fail` and then remove it with `mdadm --remove` before adding a replacement. Without removing the failed device, the array still considers the old disk as part of the array, and the new disk is not recognized as a replacement, so no rebuild starts.
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.
✗
The new disk must be partitioned with the same layout as the failed disk.
Why it's wrong here
mdadm works on whole disks or partitions; superblock is used.
✗
The --add command should have been --re-add instead.
Why it's wrong here
--re-add is for reconnecting a temporarily removed disk, not a new one.
✗
The array is still clean and does not need the new disk yet.
Why it's wrong here
If a disk failed, the array is degraded and needs rebuilding.
✓
The failed disk was not removed from the array first; you need to mark it as failed and remove it.
Why this is correct
Use mdadm --manage /dev/md0 --fail /dev/sdX and --remove before adding new disk.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 a failed disk is automatically removed from the array, but mdadm requires explicit removal before a new disk can be added to trigger a rebuild.
Detailed technical explanation
How to think about this question
In mdadm, when a disk fails, the array enters a degraded state and the failed disk is marked as faulty in the superblock. The `--add` command only works if the failed device has been removed from the array; otherwise, the new disk is treated as a spare. The proper sequence is: `mdadm /dev/md0 --fail /dev/sdc_old`, then `mdadm /dev/md0 --remove /dev/sdc_old`, then `mdadm /dev/md0 --add /dev/sdc`. This ensures the array recognizes the new disk as a replacement and begins the resync/recovery process automatically.
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.
Configure local storage — This question tests Configure local storage — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The failed disk was not removed from the array first; you need to mark it as failed and remove it. — Option D is correct because when a disk in a RAID array fails, the array marks it as faulty but does not automatically remove it. The administrator must first explicitly mark the failed disk as failed with `mdadm --fail` and then remove it with `mdadm --remove` before adding a replacement. Without removing the failed device, the array still considers the old disk as part of the array, and the new disk is not recognized as a replacement, so no rebuild starts.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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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