I've been running OMV on my Thecus N5550 successfully for a couple of years now. Only recently whenever I boot the box, my RAID5 setup ends up degraded with a disk (sdb) missing from it's configuration. I can 'fix' it by adding the disk back with mdadm --manage /dev/md0 --add /dev/sdb . Unfortunately it then apparently needs to rebuild, but it will be online again after it's done.
Of course this is not ideal. What could be the cause?