NEC Express5800 320La System Configuration Manual page 17

A server configured as RAID Level 1 with disks in the paired slots is shown in
the Figure below.
!
SLOT1 - SLOT4
!
SLOT2 - SLOT5
!
SLOT3 - SLOT6
SLOT1
SLOT2
SLOT3
Note:
Paired hard disk drives in a RAID Level 1 configuration must
have the same capacity and the same logical structure. Using the
ftdiskadm utility to manage your disks and disk partitions, ensures these
rules.
Slots for mirroring
Group 1 Group 2
SLOT4
SLOT5
SLOT6
Configuring Linux 2-9