How long does it take to rebuild a 20TB drive in a NAS?

In short

As a theoretical floor, a 20TB drive read or written flat-out at 180–260 MB/s sustained takes roughly 21–31 hours. Real rebuilds run considerably longer under parity, concurrent use and throttling, and SMR can multiply it. At this capacity use RAID 6 / SHR-2 and CMR drives, and start the rebuild immediately.

Last verified July 22, 2026 · 2 official sources checked Updated 2 months ago

Rebuild-time floor estimator

Drive capacity (TB)Sustained rate (MB/s)

Theoretical floor: 27 h 47 m

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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At 20 TB a rebuild routinely spans more than a day, and the multi-day tail is common under real load: the strongest argument on the site for double parity.

The theoretical floor

Capacity ÷ sustained transfer rate is the fastest possible rebuild. Current 3.5-inch NAS drives publish sustained rates around 180–260 MB/s. A 20 TB drive: 20,000,000 MB ÷ (180–260 MB/s) ≈ 21–31 hours as a floor. The estimator above lets you try your drive's rate.

Why real rebuilds run longer

Parity computation, concurrent use and throttling push well past the floor.
SMR drives can stretch a 20 TB rebuild into many days.

Why it matters

A single-parity 20 TB rebuild can leave you unprotected for over a day: long enough that a second failure is a real possibility. At this capacity, RAID 6 / SHR-2 is the sensible default. Use CMR drives (an SMR replacement here is a genuine hazard) and start the repair at once. The table lists in-stock 20 TB CMR drives by price per terabyte.

20 TB CMR drives, by CA$/TB

Live Amazon.ca prices in CAD, ranked by price per terabyte. The Recording column is verified against manufacturer documentation. Prices checked October 7, 2026

Model	Capacity	Recording	Price	CA$/raw TB
Seagate Exos X24 ST20000NM002H 20TBNewbest price per TB CMR recording, SATA enterprise drive Amazon.ca · sold by THEKEYKEY · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026 CA$1,165Buy on Amazon	20 TB	CMR ✓	CA$1,165	CA$58.25/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, 2026 CA$1,235Buy on Amazon	20 TB	CMR ✓	CA$1,235	CA$61.75/TB	Buy on Amazon
Toshiba N300 PRO 20TB 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 - HDWG62AXZSTBNew CMR recording, SATA NAS drive rated for 24/7 use Amazon.ca · sold by OneDealOutlet Canada · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026 CA$1,366Buy on Amazon	20 TB	CMR ✓	CA$1,366	CA$68.31/TB	Buy on Amazon
Western Digital 20TB WD Red Pro NAS Internal Hard Drive HDD - 7200 RPM, SATA 6 Gb/s, CMR, 512 MB Cache, 3.5" - WD202KFGXNew CMR recording, SATA NAS drive rated for 24/7 use Amazon.ca · sold by Amazon.ca · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026 CA$1,545Buy on Amazon	20 TB	CMR ✓	CA$1,545	CA$77.25/TB	Buy on Amazon
Western Digital Ultrastar WUH722020CLE6L4 20 TB Hard Drive - 3.5" Internal - SATA - Energy-Assisted Magnetic Recording (EAMR) MethodNew CMR recording, SATA enterprise drive Amazon.ca · sold by Rarewaves-CA · one bare internal drive; shipping and taxes not included · checked Oct 7, 2026 CA$1,862Buy on Amazon	20 TB	CMR ✓	CA$1,862	CA$93.10/TB	Buy on Amazon