You can simply remove the drive. Taking it offline first is just "best practice"; there is obviously no warning when a drive fails for real. As for the disconnect between OMSA and hardware ... make sure drivers, firmware, and OMSA are all up to date.
This drive keep going offline by itself, the controller rebuilds.  I have a new drive, but when I take the Offline... Execute option in the Administrator it goes to a blank page. I come back to page and it is still online. It is RAID 10 with 4 disks and two spans.  The Properties page say no Failure Predicted by the drive - by itself goes offline once a day. Thanks,  
PowerEdge 2950 drive cannot take offline.