I purchased two Rosewill RX307-PU3-35B disk enclosures. I can add one to the array, however, when i select the second from the dropdown, it will not select it. 

 

Instead, I get the following error in my syslog: 

 

Error: disk3: 111122223333-0:0 already assigned

 

Both enclosures have different sized disks in them, and work if i try only one at a time.

 

Any thoughts?

Since you didn't post your diags I'm guessing both disks appear to unRAID as 111122223333-0:0 , so for all purposes they look to unRAID like they are the same disk, if that's the case you'll need to replace one of the enclosures with another from a different brand/model.