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.
A server has a software RAID 5 array /dev/md0. One of its disks fails. The administrator wants to replace it without rebooting. Which command should be used to mark the disk as failed?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "which command"
Why it matters: Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
mdadm --fault /dev/md0 /dev/sdb
Why wrong: --fault is incorrect.
echo faulty > /sys/block/md0/md/dev-sdb/state
Why wrong: While possible, it's not the standard mdadm command.
mdadm --set-faulty /dev/md0 /dev/sdb
Why wrong: --set-faulty is not a valid option.
mdadm --fail /dev/md0 /dev/sdb
Correct: mdadm --manage with --fail.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
mdadm --fail /dev/md0 /dev/sdb
The correct command to mark a disk as failed in a software RAID array without rebooting is `mdadm --fail /dev/md0 /dev/sdb`. This command tells the md driver to mark the specified disk as faulty, which triggers the RAID 5 array to degrade and allows the failed disk to be removed and replaced while the system remains online.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
mdadm --fault /dev/md0 /dev/sdb
Why it's wrong here
--fault is incorrect.
echo faulty > /sys/block/md0/md/dev-sdb/state
Why it's wrong here
While possible, it's not the standard mdadm command.
mdadm --set-faulty /dev/md0 /dev/sdb
Why it's wrong here
--set-faulty is not a valid option.
mdadm --fail /dev/md0 /dev/sdb
Why this is correct
Correct: mdadm --manage with --fail.
Clue confirmation
The clue word "which command" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
The trap here is that candidates confuse the valid `--fail` option with the non-existent `--fault` or `--set-faulty` options, or they incorrectly think the sysfs method is the standard command-line approach expected in the EX200 exam.
Trap categories for this question
Command / output trap
While possible, it's not the standard mdadm command.
Detailed technical explanation
Under the hood, `mdadm --fail` sends an ioctl to the md driver, which updates the RAID superblock on the remaining disks to mark the failed disk as faulty. In a RAID 5 array, this triggers a degraded state where parity is recalculated from the remaining disks; the failed disk can then be removed with `mdadm --remove` and a new disk added with `mdadm --add`, initiating a recovery process that rebuilds the array online.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
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FAQ
Configure local storage — This question tests Configure local storage — Read the scenario before looking for a memorised answer..
The correct answer is: mdadm --fail /dev/md0 /dev/sdb — The correct command to mark a disk as failed in a software RAID array without rebooting is `mdadm --fail /dev/md0 /dev/sdb`. This command tells the md driver to mark the specified disk as faulty, which triggers the RAID 5 array to degrade and allows the failed disk to be removed and replaced while the system remains online.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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