How long does it take to rebuild a 4TB drive in a NAS?

There is no single honest number, so start with the arithmetic and then add the real-world caveats.

The theoretical floor

A rebuild has to read or write across the whole drive. Dividing capacity by a drive's sustained transfer rate gives an absolute floor: the fastest it could possibly go with nothing else happening. Current 3.5-inch NAS drives publish sustained rates of roughly 180–280 MB/s on their datasheets. So a 4 TB drive: 4,000,000 MB ÷ (180–280 MB/s) ≈ 4–6 hours as a floor. Use the estimator above to try other rates.

Why real rebuilds run longer

• Parity computation: RAID 5/6 and SHR reconstruct data from parity, which is extra work beyond a flat copy.
• Concurrent use: if you keep using the NAS during the rebuild, it competes for the drives and slows down.
• Throttling: the NAS may cap rebuild speed to keep the array responsive.
• SMR drives: a shingled replacement can multiply the time dramatically, or stall.

Why it matters

While a single-parity array rebuilds it has no remaining fault tolerance, so a second failure during the window loses the pool. That is the reason to start the rebuild immediately, keep load light, and use CMR drives. On large arrays, double parity (RAID 6 / SHR-2) survives a second failure through the rebuild.

The table lists in-stock 4 TB-and-up CMR drives by price per terabyte.