On raspberry pi 4, I moved the root filesystem from the partition on the sd card to the hard disk (changing the PARTuuId in the boot command and in /fstab). I left the boot partition on the sd card and deleted the root filesystem partition.
When I booted everything was fine and I was able to browse the OMV gui.
Since I had two sd cards, I decided to do the same on the second one (I couldn't clone from the first one since it was larger than the second).
However when I tried to boot from this second card, although everything else was fine, I could not browse and got error 'nginx failed 97 address family not supported by protocol'.
As far as I can see the boot partition contains exactly the same files on both sd cards, and they obviously use the same filesystem on the hard disk.
I really can't understand why the two cards behave differently.