A four-bay NAS of odd disks, ruined by its own rebuild
Volume Crashed was on the screen. Overnight the DiskStation had tried to rebuild onto a member that was already dropping sectors of its own. The four disks were all different sizes, which is why SHR had laid the array across more than one mdadm set, and LVM and btrfs then sat on top; pull any single disk and it tells you nothing. Every one of the four was imaged, the layers reassembled in order, and the shares mounted.