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220-1201 Practice Question: A technician is configuring a new NAS with four…
A technician is configuring a new NAS with four 10 TB hard drives. The customer wants maximum usable capacity with fault tolerance for one drive failure. The technician sets up RAID 5, but after initialization, the usable capacity is only 10 TB instead of the expected 30 TB. What is the most likely cause?
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
✓
The array was accidentally configured as RAID 1 (mirroring) across two drives.
RAID 5 with four drives of equal size should yield (n-1) * capacity = 30 TB. A 10 TB result suggests only two drives are being used, possibly due to a misconfiguration like RAID 1 or a stripe set of two drives. This tests RAID 5 capacity calculation and troubleshooting.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
One of the drives is defective and the array is using only three drives.
Why it's wrong here
If one of the four 10 TB drives were defective, the array would effectively operate with three drives. Assuming a common RAID level for three drives, such as RAID 5, one drive's capacity would be allocated for parity information, leaving two drives for data storage. With 10 TB drives, this configuration would yield 20 TB of usable capacity (2 x 10 TB), which does not match the reported 10 TB. Therefore, a single defective drive leading to 10 TB usable capacity is an incorrect deduction.
- ✓
The array was accidentally configured as RAID 1 (mirroring) across two drives.
Why this is correct
Configuring the array as RAID 1 (mirroring) across two of the 10 TB drives would indeed result in a total usable capacity equal to the size of a single drive, which is 10 TB. In this setup, one 10 TB drive serves as an exact duplicate of the other 10 TB drive, providing data redundancy. The remaining two drives would either be unused, configured in a separate array, or potentially part of a different RAID volume, but the specific 10 TB usable capacity precisely matches a two-drive RAID 1 configuration.
- ✗
The drives are not all the same size; one is 5 TB.
Why it's wrong here
If the NAS contained four drives, with three being 10 TB and one being 5 TB, and a RAID configuration like RAID 5 or RAID 10 were implemented, the usable capacity would be limited by the smallest drive's size. For instance, in a RAID 5 array, if all drives were treated as 5 TB, the total usable capacity would be (4-1) * 5 TB = 15 TB. This outcome of 15 TB does not align with the reported 10 TB usable capacity. A single 5 TB drive would reduce the overall capacity, but 10 TB is an unlikely result for four drives, even with one being smaller, unless only two drives were actually utilized in a specific RAID 1 setup.
- ✗
The RAID controller is using a different stripe size.
Why it's wrong here
Stripe size, also known as block size or chunk size, defines the amount of data written to a single disk before the RAID controller moves to the next disk in the array. While stripe size significantly influences the performance characteristics of the RAID array, affecting read/write speeds for various data types, it has absolutely no impact on the total usable storage capacity. The overall capacity is solely determined by the number and individual sizes of the physical drives, combined with the specific RAID level implemented, making this option irrelevant to the capacity issue.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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