NAS, RAID and Backup Storage
Drive failure rate to array risk.
With an annualized failure rate (AFR) of 1.41% per drive, the chance a given drive fails within 5 years is 6.85%. For 4 drives, that is a 24.72% chance that at least one fails and a 2.57% chance that more than one fails if failed drives were never replaced. Runs in your browser; nothing is sent or stored.
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Worked example
AFR 1.41% means each drive has a 1.41% chance of failing per year. Over 5 years that is 1 - (1 - 0.0141)^5 = 6.85% per drive. For 4 drives the chance that at least one fails is 24.72% (about 0.27 drives expected), and the chance that more than one fails, which RAID 5 cannot survive if no drive is replaced, is 2.57%. If a rebuild takes 24 hours, the chance that one of the other 3 drives fails during it is about 0.012%.
How it works
Annualized failure rate is failures per drive-year. Backblaze publishes AFR with drive days: its Q2 2026 figures of 1,498 failures over 31,553,350 drive days work out to 1,498 / (31,553,350 / 365) = 1.73%. We convert AFR to a constant hazard rate, so the chance one drive fails in t years is 1 - (1 - AFR)^t. For n drives the number of failures follows a binomial distribution, and the array fails when more drives fail than it can lose.
This is a planning estimate, not a prediction. Real failure rates change with drive age, model and workload; Backblaze's fleet is data-center drives, not a home NAS. Failures are also not fully independent: one study cited in Wikipedia found only 20 to 55% of storage system failures were due to disks, with correlated failures from other parts. Pair this with the RAID rebuild time calculator and the URE risk calculator.
Estimates only. RAID is not a backup: it keeps an array running through a drive failure but does not protect against deletion, malware, fire or theft. Real arrays and drives vary, so use this for planning and check your own drive datasheets.
Sources and basis (opened 2026-10-08):
- Hard Drive Reliability & Test Data (Backblaze): Q2 2026 snapshot of 31,553,350 drive days, 1,498 drive failures and a 1.73% AFR; annual 2025 of 115,638,676 drive days, 4,317 failures and 1.36%; lifetime of 558,148,607 drive days, 21,609 failures and 1.41%. We checked that the published percentages match failures / (drive days / 365).
- RAID (Wikipedia): Correlated disk failures (Jiang et al., 2008: only 20 to 55% of storage system failures were due to disks) and the increasing chance of a second failure during long rebuilds.
FAQ
What is annualized failure rate?
The expected number of drive failures per drive-year, shown as a percentage. The Backblaze figures we checked match failures / (drive days / 365).
How do I turn AFR into the chance of failure over 5 years?
Use 1 - (1 - AFR)^years. With 1.41% that is 6.85% for one drive over 5 years.
What are the odds two drives fail?
It depends on how many drives you have and how fast you replace failed ones. For 4 drives at 1.41% over 5 years, the chance that more than one fails with no replacement is 2.57%. The chance of a second failure during a 24 hour rebuild is about 0.012%.
Is the Backblaze AFR right for my NAS?
Treat it as a reference point only. Backblaze runs data-center drives in its own environment, and AFR varies with model and age. Use the rate for your drive model if you have it.
Does a low failure chance mean I can skip backups?
No. Drive failure is only one way to lose data. Deletion, malware, fire, theft and correlated failures are not covered by RAID, so keep separate backups.