Independent Australian Storage & Infrastructure Authority

RAID Rebuild Risk Calculator. How Safe Is Your Data?

This RAID rebuild risk calculator estimates the probability of a second drive failure during a rebuild window based on drive count, size, age, and RAID level. Quantifies the data loss risk that exists between the moment a drive fails and the rebuild completes.

When a drive fails in a RAID array, the rebuild process stresses every remaining drive. If a second drive fails during the rebuild, or produces an unrecoverable read error, you lose all your data. This calculator estimates that risk based on your array type, drive age, size, and specification.

Array Configuration

Rebuild Risk Assessment

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Estimated rebuild time
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URE probability
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Drive AFR estimate

How we calculate this

We estimate two independent risks and combine them: the chance another drive fails during the rebuild window (based on published/field annual failure rates for that drive age and class), and the chance of an unrecoverable read error (URE) while reading the surviving drives (based on each drive class's rated bit error rate). For RAID 5 and RAID 1, either event on its own causes data loss, since there's no remaining redundancy once the first drive has failed. RAID 6 retains one spare layer of parity after the first failure, so it only fails if a second fault (another drive failure, or a URE) occurs during the same rebuild. Reference URE rates use the more conservative figure within each drive category (e.g. "NAS HDD" uses WD Red Plus's published rate rather than IronWolf's better one) so the estimate doesn't understate risk for any specific drive in that group.

Related Tools & Guides

RAID Usable Capacity Calculator → RAID Rebuild Time + Risk Estimator → Best NAS Hard Drives (AU) → 3-2-1 Backup Strategy Guide → All Tools →

Frequently Asked Questions

What is a URE and why does it matter during RAID rebuild?
An Unrecoverable Read Error (URE) happens when a drive returns an error instead of data during a read. Consumer HDDs and budget NAS drives (WD Red Plus) have a URE rate of approximately 1 in 10^14 bits read (about 12.5 TB of data); enterprise and enterprise-NAS drives (IronWolf, Exos) rate roughly 10× better at 1 in 10^15. During a RAID 5 or RAID 1 rebuild, a URE on any surviving drive causes total data loss, since there's no spare redundancy left once the first drive has failed. RAID 6 tolerates a single URE during rebuild (it still has one spare parity layer), but not a URE combined with a second drive failure. On a 4-drive RAID 5 array with 4 TB drives, the rebuild reads approximately 12 TB, close to the URE threshold for the more conservative drive classes, which is why RAID 5 risk climbs quickly with array size on budget hardware.
What is AFR (Annual Failure Rate)?
AFR is the percentage probability that a drive will fail within a 12-month period. Consumer drives typically rate around 0.5-1.5% AFR when new, rising to 3-5% after 3-4 years (the "bathtub curve", early failures drop off, mid-life is stable, then wear-out failures increase). NAS drives like WD Red and Seagate IronWolf are rated 0.5-1.0% AFR over their lifespan. Enterprise drives like IronWolf Pro and Seagate Exos typically achieve 0.35-0.5% AFR. Backblaze publishes real-world AFR data annually, a useful cross-reference.
Is RAID 6 / SHR-2 worth the extra cost for large arrays?
For arrays with 6+ large drives (8 TB+), RAID 6 is strongly recommended. RAID 5 / SHR-1 can tolerate one drive failure; if a second drive fails during rebuild, or a URE occurs anywhere in the read, you lose everything. RAID 6 / SHR-2 can tolerate two simultaneous faults, so after the first drive has already failed it can still absorb one more full drive failure or one URE, whichever comes first, before data is lost. It's a second full-drive failure combined with a URE (or two more full failures) that a RAID 6 rebuild can't survive. With modern large-capacity drives, rebuild times can exceed 20-40 hours, during which this combined risk is non-trivial. The cost of an extra parity drive is small insurance compared to re-buying drives, NAS hardware, and restoring from backup.
What should I do immediately when a drive fails in my RAID array?
1. Do NOT power cycle unless absolutely necessary, this stresses all drives. 2. Identify the failed drive from your NAS admin interface. 3. Check the health of all remaining drives via SMART data, look for reallocated sectors, pending sectors, or uncorrectable errors as warning signs. 4. Order a replacement drive (same size or larger, same or better spec). 5. Do NOT rebuild until you have a current backup of your data. 6. Once replacement arrives, insert and initiate rebuild, monitor temperatures during the process. 7. After rebuild, run a RAID consistency check. 8. Consider replacing all remaining drives if they are the same batch and age.
Does Synology SHR behave the same as RAID 5 for risk purposes?
Yes: Synology Hybrid RAID (SHR) with one parity drive is equivalent to RAID 5 for rebuild risk. SHR-1 tolerates one drive failure; SHR-2 tolerates two. The main advantage of SHR over standard RAID 5 is that SHR handles mixed-size drives efficiently (useful when expanding gradually). The underlying failure risk model is identical. This calculator treats SHR-1 = RAID 5 and SHR-2 = RAID 6 for all calculations.