Quick Answer
The correct command is `mdadm --manage /dev/md0 --fail /dev/sdb --remove /dev/sdb --add /dev/sdc`. This sequence is necessary because mdadm requires you to explicitly mark the failed disk with `--fail` before removing it with `--remove`, and then the replacement disk is added with `--add`; skipping any of these steps can leave the array in an inconsistent state or fail to register the new disk. On the Red Hat Certified System Administrator EX200 exam, this task tests your understanding of managing degraded software RAID arrays under real-world conditions, often appearing as a performance-based item where you must type the exact command. A common trap is attempting to remove the failed disk without first failing it, which mdadm will reject. Remember the mnemonic "Fail, Remove, Add" in that strict order—never skip the fail step, as it tells the kernel to stop using that device and allows the array to continue operating degraded.
This EX200 practice question tests your understanding of configure local storage. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 has created a RAID 1 array using mdadm with two 1TB disks. After a disk failure, the array is in a degraded state. Which command should be used to replace the failed disk with a new one?
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 --add /dev/md0 /dev/sdc
Why wrong: Must fail and remove the old disk first.
mdadm --manage /dev/md0 --fail /dev/sdb --remove /dev/sdb --add /dev/sdc
Correct sequence: fail, remove, add.
mdadm --remove /dev/md0 /dev/sdb --add /dev/sdc
Why wrong: Must fail the disk before removing.
mdadm --replace /dev/md0 --with /dev/sdc
Why wrong: --replace is not a valid mdadm option.
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 --manage /dev/md0 --fail /dev/sdb --remove /dev/sdb --add /dev/sdc
Option B is correct because it uses the `--manage` subcommand to first mark the failed disk (`/dev/sdb`) as failed with `--fail`, then remove it with `--remove`, and finally add the replacement disk (`/dev/sdc`) with `--add`. This is the proper sequence in mdadm to replace a failed disk in a RAID 1 array while the array is degraded.
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 --add /dev/md0 /dev/sdc
Why it's wrong here
Must fail and remove the old disk first.
mdadm --manage /dev/md0 --fail /dev/sdb --remove /dev/sdb --add /dev/sdc
Why this is correct
Correct sequence: fail, remove, add.
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.
mdadm --remove /dev/md0 /dev/sdb --add /dev/sdc
Why it's wrong here
Must fail the disk before removing.
mdadm --replace /dev/md0 --with /dev/sdc
Why it's wrong here
--replace is not a valid mdadm option.
Common exam traps
The trap here is that candidates often think they can simply add a new disk with `--add` or remove the old disk directly without first marking it as failed, but mdadm requires the explicit `--fail` step to safely replace a failed disk in a degraded array.
Detailed technical explanation
Under the hood, mdadm manages RAID arrays via the `/dev/md*` device and kernel MD driver. When a disk fails, the array enters a degraded state and continues operating with reduced redundancy. The `--fail` action updates the superblock to mark the disk as faulty, `--remove` detaches it from the array, and `--add` triggers a recovery process that rebuilds the array onto the new disk. In a real-world scenario, if you skip `--fail` and directly remove a disk that has not been properly failed, the kernel may still consider it part of the array, leading to I/O errors or a failure to re-add the new disk.
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 --manage /dev/md0 --fail /dev/sdb --remove /dev/sdb --add /dev/sdc — Option B is correct because it uses the `--manage` subcommand to first mark the failed disk (`/dev/sdb`) as failed with `--fail`, then remove it with `--remove`, and finally add the replacement disk (`/dev/sdc`) with `--add`. This is the proper sequence in mdadm to replace a failed disk in a RAID 1 array while the array is degraded.
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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Same concept, more angles
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. 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?
hardWhy D: 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.
Variation 2. An administrator suspects a disk failure in /dev/md0. Which command would be used to simulate a failure on /dev/sdb and then remove it from the array?
hardWhy C: Option C is correct because the `mdadm` command with the `-f` (fail) and `-r` (remove) flags in a single command line first marks /dev/sdb as faulty in the /dev/md0 array, then immediately removes it. This is the standard syntax for simulating a failure and removing the disk in one step, as tested in the EX200 exam.
Last reviewed: Jun 11, 2026
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