Why is my RAID rebuild taking days?
Usually one of: very large drives (rebuild time scales with capacity), heavy use of the NAS during the rebuild, a struggling drive with slow reads, or — most commonly for a stall — an SMR replacement drive, whose write speed collapses under sustained rebuild writes. Large arrays of big drives simply take a long time; an SMR drive can make it endless.
Last verified July 22, 2026 · 2 official sources checked Updated
This is a floor, not a promise: it assumes the drive reads or writes flat-out at the rate you set, with nothing else happening. Real rebuilds run longer — parity computation, using the NAS during the rebuild, and drive throttling all add time, and an SMR drive can multiply it. Sustained rates for current 3.5-inch NAS drives run roughly 180–280 MB/s (see each drive’s datasheet); 200 MB/s is a reasonable middle default.
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A multi-day rebuild has a few usual causes. Some are normal (big drives take time); one is a red flag (an SMR drive).
Normal, if slow
- Large capacity: rebuild time scales with drive size — a 16–20 TB array legitimately takes many hours to over a day.
- Active use: if the NAS is serving files or running apps hard during the rebuild, it competes for the drives and slows the repair. Leaving it idle speeds it up markedly.
- RAID 6 / SHR-2: double-parity rebuilds do more work than single parity.
The stall: an SMR drive
The classic cause of a rebuild that crawls for days on a modest array is an SMR replacement drive. SMR drives rewrite overlapping zones and, under the continuous sequential writes of a rebuild, their throughput can drop to a trickle — turning a few-hour job into days, or never finishing. If you fitted a plain WD Red or another SMR drive, that is almost certainly why.
What to do
Confirm the replacement drive's model is CMR. If it is SMR, the honest fix is to swap it for a CMR drive and rebuild again. Also check the drive's health — a failing replacement with slow reads drags the rebuild too. Leave the NAS idle while it rebuilds. The table lists CMR NAS drives.
Drives that solve this
Live Amazon.ca prices in CAD, ranked by price per terabyte. The Recording column is verified against manufacturer documentation.
Related questions
Sources
Every technical claim on this page is checked against manufacturer and standards documentation. Dates are when each source was last reviewed.
- Western Digital — Internal drives CMR/SMR statement — checked 2026-07-22
- Synology KB — Repair a Storage Pool — checked 2026-07-22