Hello,  If a drive has been marked as failed, even if it comes back online we mark it as "History of Failure" so the array won't ever use it again by mistake.  So I would replace that one also.   Don 
One of array's disks failed and as I can see from even log, system was trying to reconstruct data to spare disk but wasn't able to complete and by some reason this spare disk failed too (at least it is stated like that in Event log). First failed disk was showing appropriate LED (one OFF, one ORANGE) while the second failed drive showing both LED green ON. Status of the first disk was "failed" but the second drive was "offline". I have already replaced the first disk and it is online and operational but the second one is still showing "offline" and both LEDs are green. Event log shows warning - "more spare drives are expected". Does that mean I have to replace this disk too? Thanks. 
Disk is "offline" in PS4100 array