Can I mix SMR and CMR drives in a NAS?

Mixing is possible but self-defeating in a RAID array. The array is only as dependable as its weakest member during a rebuild, and an SMR drive is the weak member.

The rebuild problem

When the array rebuilds, after a failure or an expansion, every drive writes continuously for hours. An SMR drive's throughput collapses under that load, so it becomes the bottleneck: the rebuild slows to the SMR drive's pace and can stall. In a mixed array you get the capacity of both but the rebuild reliability of the SMR one, which defeats the point of RAID.

If you already have an SMR drive

Do not put it in the RAID pool. Instead use it as a standalone volume: a Basic (single-drive) volume for backups or media you can re-download, where sustained writes are rare and a rebuild never happens. That plays to SMR's strengths (cheap capacity) and avoids its weakness.

For the array

Use all-CMR drives, ideally the same capacity. The table lists in-stock CMR NAS drives by price per terabyte so you can match your existing array.