If the data is critical and you don't have a backup, get one now. NEVER shut down a server to introduce a drive. If the drive is showing pred fail, force the drive offline first, then remove it. If it is already failed, then just pull it out.
Having never had to do replace a HDD in a RAID 1 configuration on a PowerEdge 2950 server (PERC 5/i). Do I do it warn or from a cold boot? I am replacing the failing HDD with the new HDD with the same part number. It is my boot volume so I really don;t want to screw it up.... Thx 
Replacing a HDD in RAID 1 PowerEdge 2950, question on process