How much space will you get?
Pick your drives and a layout, and it'll tell you what's left after parity.
SHR · 4 drives · 40 TB raw
28TB
usable
≈ 25.5 TiB, which Windows calls “TB”
Survives 1 drive failing
- Usable
- 28 TB
- Protection
- 12 TB
- Unused
- 0 TB
Compare all layouts on these drives
Rules of thumb
- −1 driveSHR and RAID 5 give one drive's worth of space to parity.
- −2 drivesSHR-2 and RAID 6 give up two drives' worth, which is what you want once drives reach 16 TB, because rebuilding a drive that big takes more than a day.
- HalfMirrors in RAID 1 or RAID 10 keep half.
- Mixed sizesSHR and Unraid use the extra space on bigger drives, while RAID 5 counts every drive as the smallest one.
Sections
- Hard drives30 NAS and data-centre drives, with speed, workload and warranty
- NAS units25 boxes from two bays up, with the CPU and ports that matter
- DAS enclosuresUSB and Thunderbolt boxes for hanging drives off a mini PC
- Cages, docks and HBAsThe bits for turning an old tower into a storage server
- Failure ratesBackblaze AFR for 36 drive models, error bars included
Lowest failure rates we list
We rank on the top of the 95% interval. A model Backblaze only runs a handful of can't sneak to the top on one lucky year.
| Drive | AFR | 95% CI |
|---|---|---|
| WD Ultrastar DC HC550 16TB84,197 drive-years | 0.54% | 0.49%–0.59% |
| WD Ultrastar DC HC570 22TB72,927 drive-years | 0.58% | 0.53%–0.64% |
| Seagate Exos X16 16TB129,390 drive-years | 0.68% | 0.63%–0.72% |
| Toshiba MG11 24TB4,642 drive-years | 0.50% | 0.31%–0.74% |
Source: Backblaze. Lifetime figures to 2026 Q2. All 36 models