What Is the Statistical Probability of a RAID 5 Rebuild Failure?
Consumer hard drives carry a manufacturer-specified Unrecoverable Read Error rate of one error per 10^14 bits read. That equals approximately 12.5 TB. Enterprise SAS drives improve this to one per 10^15 bits (~125 TB), but most NAS and small-server arrays ship with consumer SATA drives. Read that figure as a worst-case warranty floor, not a schedule: field studies (USENIX FAST latent-sector-error work, Backblaze fleet data) show the large majority of drives read far past 12.5 TB without a single URE, and read errors cluster on aging or marginal drives rather than arriving on an independent per-byte basis.
On the bench, degraded arrays rarely die from a clean per-byte bit error. They die because a full-surface parity rebuild pins every surviving member at close to 100% sustained read for 18 to 48 or more hours, and that unbroken thermal and kinetic load pushes an already-marginal head, preamp, or spindle bearing past its failure threshold mid-rebuild.
Array members also share a manufacturing batch, age, and thermal environment, so a second failure inside the rebuild window is positively correlated, not independent: a drive that has logged one scan error is far more likely to fail within the next two months than a clean drive. The clean binomial URE model overstates random bit-rot while understating this correlated mechanical risk, which is the more common real driver of a failed rebuild.
| Array Configuration | Data Read During Rebuild | Expected UREs (Consumer) | Expected UREs (Enterprise) |
|---|
| 4-drive RAID 5, 8 TB members | 24 TB | ~1.9 | ~0.19 |
| 4-drive RAID 5, 16 TB members | 48 TB | ~3.8 | ~0.38 |
| 8-drive RAID 5, 16 TB members | 112 TB | ~9.0 | ~0.90 |
What happens when a URE lands on a degraded array depends on the controller. Legacy block-level hardware RAID and low-end consumer controllers (for example Intel RST) hard abort the rebuild and drop the volume offline. HP/HPE Smart Array P-series and E-series also abort and flag POST Error 1784 or 1786. Modern Dell PERC and LSI/Broadcom MegaRAID puncture instead: they write a bad-block placeholder over the stripe, finish the rebuild, and keep the volume online, with only that stripe permanently lost. Linux mdadm records the unreadable LBA in its Bad Block Log and continues.
In every case the data inside that stripe is gone, but a single URE does not universally collapse the array. RAID 6 has more margin still: it tolerates one URE per stripe during a single-drive-down rebuild because the second parity block fills it in, where RAID 5 has no parity left to spare.
If your data is irreplaceable and you have no verified backup, do not attempt a live rebuild on degrading hardware. The rebuild reads every sector on every surviving drive under sustained stress, and a marginal same-batch survivor can fail in that window. In that high-risk, unbacked scenario we image each member through a write-blocked imager (PC-3000 Express or DeepSpar) before any reconstruction, then reassemble virtually from the cloned copies. For routine failures on arrays with verified backups, hot spares, and dual parity, a monitored controller rebuild is standard practice.
