hi all,
today finally I decided to upgrade from omv2 to omv4, windows10 has deprecated old smb v1 protocol so it convinced me to make a fresh install in my nas.
I installed from a usb media, sda, to an ssd, sdb, connected as usb (to spare a sata port) and with all old data hdd unplugged.
After rebooting without installation media, the ssd switched from sdb to sda and I faced the same problem, mdadm array etc etc.
Plug-in the data hdd (5), as suggested, not solved as os ssd it's now sdf.
The problem is in grub conf, its entry has as linux root the wrong id device (good only during installation config), in my case root=/dev/sdb1
Long story short, to get rid of the problem I pressed 'e' in the grub menu to edit parameters and I changed the root attribute with the current device so I managed to boot and then on the shell I executed "update-grub" that changed root values in uuid version 'root=UUID=...'.
I hope it could be of any aid to whom is still facing it.
Bye