- A
mdadm --add /dev/md0 /dev/sdc
Why wrong: Must fail and remove the old disk first.
- B
mdadm --manage /dev/md0 --fail /dev/sdb --remove /dev/sdb --add /dev/sdc
Correct sequence: fail, remove, add.
- C
mdadm --remove /dev/md0 /dev/sdb --add /dev/sdc
Why wrong: Must fail the disk before removing.
- D
mdadm --replace /dev/md0 --with /dev/sdc
Why wrong: --replace is not a valid mdadm option.
Question 77 of 537
EX200 Configure local storage Practice Question
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.
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
✓
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
Option-by-option breakdown
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
Common exam trap: answer the scenario, not the keyword
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
How to think about this question
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.
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.
Quick reference
RAID Level Comparison
| RAID Level | Min Disks | Fault Tolerance | Read | Write | Usable Capacity |
|---|---|---|---|---|---|
| RAID 0 | 2 | None | Excellent | Excellent | 100% |
| RAID 1 | 2 | 1 disk | Good | Moderate | 50% |
| RAID 5 | 3 | 1 disk | Good | Moderate | 67–94% |
| RAID 6 | 4 | 2 disks | Good | Lower | 50–88% |
| RAID 10 | 4 | 1 disk per mirror | Excellent | Good | 50% |
RAID is not a backup strategy — it protects against disk failure but not against accidental deletion, ransomware, or site-level events.
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.
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FAQ
Questions learners often ask
What does this EX200 question test?
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: 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.
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: "which command". Tests specific CLI syntax. Recall the exact command and its required context — near-synonyms and partial matches are common distractors.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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