How long does a Synology RAID rebuild take?
Typically several hours to a couple of days, depending on drive capacity, RAID type and how busy the NAS is. As a rough guide it scales with the size of the drives — larger drives take proportionally longer. The array stays usable during the rebuild but has reduced or no fault tolerance until it finishes.
Last verified July 22, 2026 · 1 official source 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.
As an Amazon Associate we earn from qualifying purchases on Amazon.ca.
A rebuild (Synology calls it a repair) copies parity and data onto the replacement drive to restore redundancy. There is no single number, but the drive capacity is the main driver — a rebuild reads or writes across the whole array, so bigger drives mean a longer rebuild, often measured in hours per terabyte.
What affects the time
- Capacity: the dominant factor — a 4 TB rebuild is far quicker than a 20 TB one.
- RAID type: RAID 6/SHR-2 rebuilds involve double parity and can take longer than RAID 5/SHR.
- Load: if you keep using the NAS heavily during the rebuild, it slows down; leaving it idle speeds it up.
- Drive health: a struggling drive with slow reads drags the whole rebuild.
The risk window
While rebuilding, a single-parity array (RAID 5/SHR) has no remaining fault tolerance — a second drive failing before the rebuild finishes crashes the pool. That is exactly why large arrays of big drives lean toward RAID 6 or SHR-2: the rebuild takes longer, and double parity keeps you protected through it. You can keep using the NAS during a rebuild, but avoid heavy writes and do not remove any other drive.
Speeding it up
Leave the NAS as idle as possible, ensure good cooling, and use healthy CMR drives (an SMR replacement can massively lengthen a rebuild). If a rebuild seems stuck for days on a modest array, check the replacement drive's health.
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.
- Synology KB — Repair a Storage Pool — checked 2026-07-22