Hi folks!
In my hardware setup I have four drive bays populated with data harddisks and one ssd containing the os.
While installing OMV I remove the data disks to prevent any loss of data. So at this point of time the system ssd is /dev/sda.
After initial setup I install and configure some addons needed for later operation - also sharerootfs (6.0-3)!
This is how the drives tab looks like at this point:
Sharerootfs makes the system partition (/dev/sda1 at this point) showing up on the file systems tab. Everything is fine.
Then I attach the data drives and the mess begins.
For some reasons I cannot explain the data disks are always put in front and the system disk is shifted to the end. In this case with only one harddrive the system disk becomes /dev/sdb.
As a result I get this error message in the file systems tab:
At the Dashboad the system file system is shown as "missing":
and also deisappears from the file system list:
two years ago I already faced a similar problem in OMV5 (OMV remembers 'drive letters' during installation and creates ghost file systems afterwards), but never found the reason nor solution to that.
Back then I had no idea that sharerootfs might be responsible for those "missing" ghost entries in the OMV5 file systems tab but I'm confident now that sharerootfs in combination with my system shifting the "driveletters" when attaching more drives is the problem.
When I realized that I uninstalled and reinstalled sharerootfs and everything went back to normal!
So votdev : I have no idea what sharerootfs does (and WHEN it does it) but it is obviously overwhelmed when a newly attached drive pushes the system drive back in the sequence.
Reproducing this behavior in Virtualbox is a bit hard but I managed to do so.
First install OMV on a single virtual hard drive attached to SATA port 0 (default).
Then in OMV install sharerootfs and shut down the vm.
Add another virtual hard drive to the vm and reorder them so that the system disk is on SATA port 1 and the new one is on SATA port 0.
When you try to start the vm, you now get an error message (no bootable medium found).
So press F12 immediately after starting the vm and type "2" to select the right virtual hard disk for startup.
After that you'll see the exact same problem described.
I'm not a dev and I didn't look into the details of sharerootfs but I fear its magic has to be repeated every time the drive order changes to prevent this bug. 
edit: On my server setup the system ssd is set to first place in bios boot order. But nevertheless OMV pushes it behind the data disks at system start. When I attached all 4 data drives, the system ssd ends up as being /dev/sde. And every time such a shift happens these errors have to be repaired with uninstalling/installing sharerootfs again
