Hey folks,
I solved it for my situation, where I have OMV5 installed on SSD and am adding more HDDs.
The trick is not to hook up the new HDDs before the system has booted. Do as follows:
- Fire up your unchanged system
- Log into the shell (I logged in as root)
- Attach the HDD to your system (next SATA port for me) while the system is still up and running
- Run "update-grub" in the shell
- Restart your system
My system now just started up with the new HDD attached, without any mdadm message.