Adding drives to a DDP is capped at a max of 12. If you need to add more than 12 you will need to kill & fill. DDP is different than disk groups, as disk groups you can add as many drives as you want as long as they are added in pairs.    
Hi,  I found a fun new rule I didn't know about before:   [IMAGE] You can only add 12 physical disks to an existing disk pool.  Ever! That's all you get!  Why?  It doesn't say.  But wait, you actually CAN add more than 12 physical disks, it just only utilizes the capacity of the first 12, the rest is marked as "unusable".  Why?  It also doesn't say.  My best guess, because I can find nothing else on the subject, is because rebalancing would take too long / affect performance too adversely if the number of spindles grows past a certain point (after the initial creation, which appears to have no hard limits other than the max # of spindles allowed by the entire array).   Minimum of 11 disks in a pool to start, up to the max allowed by the array, but after creation, you can only add 12.  not 12 at a time, 12 ever.  I don't understand.If anyone can enlighten me further, I'd sure appreciate it.  All I wanted to do was add 17 disks to a 17 disk pool.  Found that seemingly-arbitrary rule in the manual today and frustration ensued.
MD3800i pool expansion