Why is my RAID rebuild taking days?
Usually one of four causes: very large drives (rebuild time scales with capacity), heavy use of the NAS during the rebuild, a struggling drive with slow reads, or, most often when it stalls, an SMR replacement drive, whose writes collapse under sustained rebuild load. Big arrays simply take a long time; an SMR drive can make it endless.
Last verified October 6, 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. Prices checked
| Model | Capacity | Recording | Price | CA$/raw TB | |
|---|---|---|---|---|---|
 Seagate Exos X22 22TB SATA 6Gb/s 7200RPM 3.5" Enterprise Hard Drive - ST22000NM001ENewbest price per TB CMR recording, SATA enterprise drive Amazon.ca · sold by ServerPartDeals · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$1,108Buy on Amazon | 22 TB | CMR ✓ | CA$1,108 | CA$50.38/TB | Buy on Amazon |
 Seagate IronWolf Pro 32TB Enterprise NAS Internal HDD Hard Drive – CMR 3.5 Inch SATA 6Gb/s 7200 RPM 512MB Cache for RAID Network Attached Storage, Rescue Services (ST32000NTZ00)New CMR recording, SATA NAS drive rated for 24/7 use Amazon.ca · sold by THEKEYKEY · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$1,740Buy on Amazon | 32 TB | CMR ✓ | CA$1,740 | CA$54.37/TB | Buy on Amazon |
 Seagate Exos M 28TB Enterprise Internal Hard Drive HDD - 3.5in 6GB/s SATA 7200RPM 2.5M MTBF (ST28000NM003K)New CMR recording, SATA enterprise drive Amazon.ca · sold by Amazon.ca · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$1,543Buy on Amazon | 28 TB | CMR ✓ | CA$1,543 | CA$55.11/TB | Buy on Amazon |
 Seagate IronWolf 10TB NAS Internal Hard Drive HDD – CMR 3.5 Inch SATA 6Gb/s 7200 RPM 256MB Cache for RAID Network Attached Storage (ST10000VN0008) (ST10000VNZ008/N0008)New CMR recording, SATA NAS drive rated for 24/7 use Amazon.ca · sold by Deals2You · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$579Buy on Amazon | 10 TB | CMR ✓ | CA$579 | CA$57.91/TB | Buy on Amazon |
 Seagate Exos X24 ST20000NM002H 20TBNew CMR recording, SATA enterprise drive Amazon.ca · sold by THEKEYKEY · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$1,165Buy on Amazon | 20 TB | CMR ✓ | CA$1,165 | CA$58.25/TB | Buy on Amazon |
 Western Digital HUH721212ALE604 12TB Ultrastar DC HC520 SATA HDD - 7200 RPM Class, SATA 6 Gb/s, 256MB Cache, 3.5"New CMR recording, SATA enterprise drive Amazon.ca · sold by Seller_King · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$699Buy on Amazon | 12 TB | CMR ✓ | CA$699 | CA$58.25/TB | Buy on Amazon |
 Seagate Exos X24 24TB Enterprise Internal Hard Drive HDD - 6GB/s SATA 7200RPM 2.5M MTBF (ST24000NM002H)New CMR recording, SATA enterprise drive Amazon.ca · sold by THEKEYKEY · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$1,400Buy on Amazon | 24 TB | CMR ✓ | CA$1,400 | CA$58.33/TB | Buy on Amazon |
 Toshiba N300 PRO 8TB Large-Sized Business NAS (up to 24 Bays) 3.5-Inch Internal Hard Drive - Up to 300 TB/Year Workload Rate CMR SATA 6 GB/s 7200 RPM 512 MB Cache - HDWG780XZSTBNew CMR recording, SATA NAS drive rated for 24/7 use Amazon.ca · sold by HYPER MEGA Electronics · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$480Buy on Amazon | 8 TB | CMR ✓ | CA$480 | CA$60.00/TB | Buy on Amazon |
 Seagate IronWolf Pro 20TB Enterprise NAS Internal HDD Hard Drive – CMR 3.5 Inch SATA 6Gb/s 7200 RPM 256MB Cache for RAID Network Attached Storage, Rescue Services (ST20000NTZ01/ST20000NT001)New CMR recording, SATA NAS drive rated for 24/7 use Amazon.ca · sold by THEKEYKEY · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$1,235Buy on Amazon | 20 TB | CMR ✓ | CA$1,235 | CA$61.75/TB | Buy on Amazon |
 Western Digital 22TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD221KFGXNew CMR recording, SATA NAS drive rated for 24/7 use Amazon.ca · sold by Deals2You · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026CA$1,400Buy on Amazon | 22 TB | CMR ✓ | CA$1,400 | CA$63.64/TB | Buy on Amazon |
Related questions
- Can SMR drives be used in RAID?
- How long does a Synology RAID rebuild take?
- How long does it take to rebuild a 16TB drive in a NAS?
- Should I upgrade to RAID 6 for large drives?
- How do I tell if my hard drive is SMR or CMR?
- Is the WD Red 4TB SMR or CMR?
- Can I mix SMR and CMR drives in a NAS?
- How long does it take to rebuild an 8TB drive in a NAS?
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)