I just did a fresh OMV5 install on an SD-card (via USB3). Everything works fine and the system boots properly (forced UEFI install, if that matters)
However, when I add any more drives to the system, the system drive can't be mounted anymore. It took me a while (and several threads in this forum) to find out the actual problem. Apparently, the system drive (/dev/sda) gets pushed back during the bootand is called sdb, sdc, sdd, ... depending on how many drives are attached.
I looked at the /etc/fstab, however the system drive is mounted via UUID, so I don't understand why OMV still talks about /dev/sda.
When I try to boot up, I get dropped into busybox. 'cat /proc/cmdline' tells me that /dev/sda2 couldn't be mounted properly.
I can still mount the system drive (now 'sdd2'), and look at the fstab. Obviously I can tell the fstab to try mounting /dev/sdd2 instead of the UUID, but that's just a temporary solution, since it'll be pushed off to 'sde2' if I ever plug in another drive.
If I shutdown the system, remove the sata drives, the system boots up properly. I can then hotplug the drives back in.