My setup on a Intel NUC6: 1 internal SSD for OMV and systemfiles like Docker. This disk is partioned: 8 GB for OMV (sda2), rest for other systemfiles (sda4). Because I am using Clonezilla (inside OMV) it is easy for me to backup and restore OMV if something is wrong or just for fun. Tried other backup methods but this one works fine for me. In the meantime I have learned not to mess about the OMV-system, took me a long time to correct errors or even restore a complete new system. Don't fix if it ain't broken
Besides the systemdisk I have 1 SSD for online data and 1 SSD for online backup (rsnapshot). Another SSD is a offline backup (USB backup). And another one for offsite backup.