Mismatched drives in a four-bay NAS, and a rebuild that made it worse
Four disks, no two of them the same size, which is why SHR had spread the volume over a pair of mdadm sets with LVM stacked on those and btrfs above that — put any one member on a bench by itself and it tells you nothing. The DiskStation had marked the volume failed before dawn, having spent the night rebuilding onto a member that was quietly losing sectors of its own. Imaging took all four, the stack went back together layer by layer, and the shares came up.