A virtual disk can go offline for varying reasons, as
for example, when more than one piece is out of service
(OOS) or when one piece is out of service and
parity is out of date (OOD).
Only RAID 1, RAID 4 and RAID 5 can go offline.
To force an offline disk back online:
Virtual Disk Manager,
select a virtual disk which is offline.
Select Disable from the Disk menu.
Select Force On-line from the Disk menu.
If the virtual disk uses parity data, it will also need restoring.
Because parity can only be used to recover data from a single disk failure,
some data may be lost.