I find “fsarchiver” very useful for system disk backup.
As I have no keyboard and monitor on my server, I boot a maintenance-linux (Ubuntu server) from a 2GB usb-key and do everything remotely via SSH.
This maintenance-linux contains among other things fsarchiver and gparted.
To archive the first partition on your system disk (sda) using fsarchiver:
assuming that you have mounted some backup-disk to /media/backup.
My OMV System Backup file is only 600 MB.
cheers, ToBi