Can I replace a drive in RAID 5 with a smaller one?

RAID 5 sizes every member to the array's per-drive size, and a replacement has to be able to hold that. A smaller drive physically cannot, so the NAS will not let you rebuild onto it.

Why smaller fails

In RAID 5 each drive stores an equal share of data and parity. If your array was built on 8 TB drives, every member holds 8 TB worth of stripes; a 6 TB replacement has nowhere to put the missing stripe, so the repair is impossible. The same applies to RAID 6 and SHR.

Same size or larger

A same-size drive restores the array exactly. A larger drive rebuilds fine and its extra capacity waits unused until every drive in the array reaches that size, at which point you can expand the volume. So sizing up is never wrong, and since bigger drives are usually cheaper per terabyte, it is often the better buy.

Recording tech still matters

Whatever the size, use a CMR drive. An SMR replacement in RAID 5 can stall the rebuild badly. The table lists in-stock CMR drives at or above common capacities, ranked by price per terabyte.