Why did my RAID rebuild fail?
Usually a second drive developed errors during the rebuild: the surviving drives are read end-to-end under load, which can expose a weak one, and on a single-parity array that crashes the pool. Other causes: an SMR or failing replacement drive, or the replacement being too small. Restore from backup and rebuild with healthy CMR drives.
Last verified July 22, 2026 · 2 official sources checked Updated
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A failed rebuild is stressful because it often means data loss, but the causes are a short list.
The most common cause: a second drive faltered
A rebuild reads every surviving drive end to end to reconstruct the missing one. That full-surface read under sustained load is exactly when a marginal second drive throws errors or drops out. On a single-parity array (RAID 5 / SHR) that has no remaining redundancy during the rebuild, a second drive failing means the pool crashes and the rebuild fails. This is the classic RAID 5 failure story, and the reason large arrays prefer RAID 6 / SHR-2.
Other causes
- SMR replacement drive: its write speed collapses under sustained rebuild writes and the rebuild stalls.
- Failing replacement: a bad new drive (check its SMART data) can abort the rebuild.
- Replacement too small: it must be at least as large as the drive it replaces.
- Power interruption: an unclean shutdown mid-rebuild, a reason to run the NAS on a UPS.
What to do now
If the pool crashed, stop writing to it, do not reinitialize, and restore from backup (or consult professional recovery for critical data with no backup). Rebuild with healthy CMR drives, and adopt RAID 6 / SHR-2 plus a UPS to avoid a repeat. The table lists in-stock CMR 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
Sources
Every technical claim on this page is checked against manufacturer and standards documentation. Dates are when each source was last reviewed.
- Synology KB: What can I do when the storage pool crashes (checked 2026-07-22)
- Synology KB: Repair a Storage Pool (checked 2026-07-22)