Manually create a RAID1 on two disks and mount the raid to /var/lib/nethserver. With this configuration you can create a server with this disks layout:
- 2 fast and small disks for the OS
- 2 bigger (and possibly slower) disks for data
More info here
http://wiki.nethserver.org/doku.php?id=howto_manually_create_raid1 by @giacomo