I have a QNAP TS-453A NAS with 4 6TB disks configured as a RAID5 storage pool. Currently with firmware 188.8.131.522.
Yesterday I discovered that one of the disks (disk 3) had failed, and that the storage pool was in a "Degraded" state.
I quickly got a new disk. The old disk was a WD Red 6TB WD60EFRX, the new one is a WD Red 6TB WD60AFEX. I hot-swapped the disks. According to the documentation, the new disk should be detected automatically, and the storage pool should automatically start rebuilding ("Rebuilding" state). But nothing happened.