How long does it take to rebuild a 20TB drive in a NAS?
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
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.
Related questions
Sources
Every technical claim on this page is checked against manufacturer and standards documentation. Dates are when each source was last reviewed.
- Seagate — IronWolf Pro datasheet (Max Sustained Transfer Rate OD, up to ~285 MB/s) — checked 2026-07-22
- Synology KB — Repair a Storage Pool — checked 2026-07-22