I'm using/testing a Microserver Gen8 with a SSD (for the OMV system) attached to port 5 - which is the last port on the on-board controller, normally used for a optical disc drive. As the BIOS doesn't allow booting off this port, I'm also using a micro SD card for a chainloader (GRUB) - which works fine so far.
At installation time i hadn't any storage disks plugged in. So the SSD became /dev/sda.
After putting in one HDD, it became /dev/sda and the SSD became /dev/sdb.
OMV seems to work fine so far - but I had problems wiping the new disk with the GUI (button was disabled for this disk, but enabled for the system disk). Also the new HDD/partition is named by device name (/dev/sda1) not by it's label in the File System view - which seems odd, at least.
Also unmounting wasn't possible (using the GUI). The table says there are still references, but I removed all shares. Something tells me this "hidden" reference was actually targeting the system disk - not the storage HDD. But since these two swapped places.... somehow I think /dev/sda is still treated differently?
As far as I can tell OMV seems to use UUIDs wherever possible (fstab etc.) - so this shouldn't be a problem. But I'm still wondering if my "moving system drive" may be a problem in the future, since I don't know at the time how many HDDs I will add or remove later (and I want to be flexible on this matter).