On a previous release of OMV I could have sworn there was an option to run OMV from the HardDrive instead of the SD card. I didn't find it on the latest release (OMV_3_0_87_Bananapi_4.12.9). In fact there were a couple of installation changes that thru me off.
Anyways I tried to do it on my own. After installing it and getting it the way I wanted, I removed the SD card and using my PC I made a backup of the root partition
cd [root partition]
tar cpjf /backup.tar.bz2 .
then I also unplugged the HD from the banana pi and using a SATA-to-USB adapter I plugged it into my PC. Then I made a small partition and extracted the backup into it. On the boot partition on the SD card I modified armbianEnv.txt and set the rootdev option to point to the new partition on the harddrive.
I put everything back to the banana pi and it seemed to run smoothly BUT I had forgotten about fstab. Using SSH, I adjusted the fstab to reflect the new UUID and rebooted.Again everything seems to run smoothly. The question is: Did I forget something else? was it that easy? I don't see errors.