Asus RS162-E4/RX4 - Manual (Page 128)

Chapter 6: RAID configuration -1
4. Use the arrow key to select the disk you would like to configure as hot
spare, then move the cursor to the Hot Spr column. Press <+>, <->, or
<Space>. The Drive Status column field now shows Hot Spare.
Press <C> to commit the changes.
Synchronizing the array
Synchronizing the array allows the utility to resynchronize data on the
mirrored disk in the array. This procedure is seldom required because data
synchronization is automatically done during normal operation.
To synchronize the array:
1. Follow steps 1 ~ 3 of the section “Viewing volume properties” and step
2 of the section “Managing hot spares.”
2. From the Manage Array screen select Synchronize Spare, then press
<Enter>.
3. Press <Y> to begin the synchronization, or <N> to cancel.