It should show the other disk.  Some things to try: Any of the above may work, but it is most likely caused by out-of-date OMSA and/or firmware/drivers.
I'm trying to make sense of the info in OMSA 2.1.0 for a RAID-1 virtual disk on a Poweredge 2850 + Perc 4e/Di.   Array disk 0:0 died at an unknown point in the past.  OMSA info prior to replacing the drive:  [IMAGE] [IMAGE]  we added another 0:0 drive in, it rebuilt successfully automatically and this is the current info:  [IMAGE]  [IMAGE]  [IMAGE]   So RAID-1 virtual disk's state is no longer 'degraded' yet it is still showing only the 0:1 Hitachi array disk and not the new 0:0 Seagate.  How should this be interpreted?  We also have a separate raid-10 virtual disk on this same server with same model drives which also had a failed drive and swapping out that drive resulted in the new Seagate drive showing up in the Raid-10 array listing as expected.  the difference was that on the Raid-10 array, the original disk was displayed in OMSA as 'failed' whereas on the above RAID-1, no other drive was listed. 
Poweredge 2850 - Perc 4e/Di - RAID-1 array status 