I just did a fresh, clean install of OMV5 on Raspberry Pi 4 with a WD external USB3 Hard Drive as storage. Everything works perfectly, as a charm!
Only issue I had, was that after a reboot, Docker became corrupt and running docker or docker-compose commands failed with weird errors about network, or layers not existing.
I think it has something to do with the docker data moved to the USB3 HD, and that the data was not available at the time of Docker init. So, to fix it, I delayed the docker initialization with 30 sec:
Note, the only important line to change is line 14, the actual delay. You also might want to increase line 28, the StartLimitInterval. I chose 300 sec.
Now, everything works as expected. After a reboot, Docker is usable without manual restart, and it also restarts your containers properly (given you have set the proper restart policies in the containers).