This article applies to the Apple and generic DAS 4600.
When a replacement hard disk drive is inserted into the unit after a disk failure, the DAS unit will not automatically rebuild to the new hard disk drive. 

You need to activate the rebuild process by accessing "Volume Configuration" menus "RAID Status List" option and selecting the replacement hard disk drive.
 
You will be prompted to "ERASE" the new drive, and after you accept this action the rebuild process will begin to the new replacement drive.

NOTE: Always refresh the SmartNAVI utility after completing a rebuild operation.