Typically somewhere in the raid software there should be a way to get the serial numbers of the drives.
I am not failure with your particular Raid system so I am guessing a bit..
In your case that may be under the Manage button.
You could try clicking on the good HD in the System storage view pane and see if that identifies the good drive serial number..
If you had a hot swap server setup you often get status lights which identify the dead drive for quickly finding the bad one..
You will need a new drive of the same size or larger, the array will rebuild with a larger drive (although you won't get more space). The replacement drive can not be smaller than the current drive size.
If those are Seagates the other drive is living on borrowed time.. I have had a bunch of 1.5TB Seagates die early deaths in a server RAID5 configuration.. You definitely will want to replace the failed drive ASAP.
