RAID Hot Spare: What It Does and How It Affects Capacity
A hot spare is an installed drive that sits idle until another drive in the array fails. The controller then rebuilds onto it automatically, without waiting for someone to swap the failed drive.
In numbers
- 200TB usable in RAID 6 from 24TB drives takes 11 drives.
- With one hot spare you buy 12 drives; usable capacity stays 216TB.
Identical drives; decimal TB as drives are sold, binary TiB as most operating systems show it; before filesystem overhead. Hot spares are extra drives and add no capacity.
Drives to buy for 200TB usable with 24TB drives
| Level | No spare | 1 hot spare | 2 hot spares | Usable |
|---|---|---|---|---|
| RAID 5 | 10 | 11 | 12 | 216TB |
| RAID 6 | 11 | 12 | 13 | 216TB |
| RAID 10 | 18 | 19 | 20 | 216TB |
| RAID 60 | 14 | 15 | 16 | 240TB |
RAID 60 shown with 2 groups. Usable capacity is the same with or without spares.
Hot spare or cold spare
A hot spare is installed and powered, and the rebuild starts on its own. A cold spare sits on a shelf: someone has to notice the failure and swap it in, and the array runs degraded until then.
Dedicated or global
A dedicated hot spare serves one array. A global hot spare serves any array on the same controller. Hardware RAID controllers such as Dell PERC and HPE Smart Array support spare drives; check your model’s documentation for the exact options.
What a spare does not do
A spare adds no capacity and does not let the array survive more simultaneous failures. It shortens the time the array runs without full protection.
24TB drives for this array
In-stock 24TB drives from our catalogue, with the price for the 11 drives in the example above. Hot spares are extra.
- Western 0F62798 Western Digital 0F62798 Ultrastar DC HC580 24TB 7200RPM 6Gb/s SATA 512e $960.00 each · 11 drives $10,560.00
- Seagate ST24000NM007H Exos X24 24TB 7200RPM 12Gb/s SAS 512MB Cache 512e 3.5-Inch Hard $1,450.00 each · 11 drives $15,950.00
- Western WUH722424ALE604 Western Digital WUH722424ALE604 Ultrastar DC HC580 24TB 7200RPM $960.00 each · 11 drives $10,560.00
- Dell GPP63 Dell 0GPP63 / GPP63 24TB 7200RPM 12Gb/s SAS 512MB Cache 512e Hot-Pluggable $1,589.47 each · 11 drives $17,484.17
- Western WD240EDGZ Western Digital WD240EDGZ 24TB 7200RPM SATA 6Gb/s 3.5-inch Internal $960.00 each · 11 drives $10,560.00
- 400-BRRZ - Dell 24TB SAS 12Gb/s 7.2K 512e 3.5-inch LFF Brand New Hard Drive for PowerEdge $1,950.00 each · 11 drives $21,450.00
- Seagate ST24000NM000H Exos X24 24TB SATA 6Gb/s 3.5-Inch Hard Drive $987.37 each · 11 drives $10,861.07
- Western 0F62835 Western Digital 0F62835 Ultrastar DC HC580 24TB 7200RPM 12Gb/s SAS 512MB $2,068.42 each · 11 drives $22,752.62
Frequently asked questions
Does a hot spare add capacity?
No. It is idle until a drive fails, then the array rebuilds onto it. Count it in the drives you buy, not in usable capacity.
Is a hot spare part of the RAID array?
Not until it is used. It sits outside the array until the controller rebuilds a failed drive onto it.
What is the difference between a global and a dedicated hot spare?
A dedicated hot spare is assigned to one array; a global hot spare can replace a failed drive in any array on the same controller.
RAID calculators by level
- RAID calculator (all levels)
- RAID 0 calculator
- RAID 1 calculator
- RAID 5 calculator
- RAID 6 calculator
- RAID 10 calculator
- RAID 50 calculator
- RAID 60 calculator
RAID planning guides
- How Many Drives Can Fail in Each RAID Level?
- TB vs TiB: Why a 24TB Drive Shows 21.8 TiB
- RAID Write Penalty by Level
- Best RAID Level for 2, 3, 4, 5, 6 or 8 Drives
- RAID 50 vs RAID 60
- RAID 6 vs RAID 60
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Updated 2026-10-22.