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
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 have a URE rate of approximately 1 in 10^14 bits read (about 12.5 TB of data). During a RAID 5 rebuild, the system must read every bit of data from all remaining drives. On a 4-drive array with 4 TB drives, the rebuild reads approximately 12 TB, exactly at the URE threshold for consumer drives. If a URE occurs during rebuild, the array fails completely and all data is lost. Enterprise drives have a URE rate 10× better (1 in 10^15 bits), dramatically reducing this risk.
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 produces a URE), you lose everything. RAID 6 / SHR-2 can tolerate two simultaneous failures. With modern large-capacity drives, rebuild times can exceed 20-40 hours, during which the risk of a second failure 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.