I have a 3300 with redundant HDDs (RAID) and I have an alarm showing a faulty hard drive. How do I determine which one it is.
You look at the LEDs on the controller. Look in the technicians handbook, there are LEDs for each drive, the bad one will have a fault/access LED combination of colors and blinking to tell you the problem.
If you replace one, it must be the EXACT same drive, same part number, revision, etc... if you replace it with a slightly different drive it won't work. Typically I suggest replacing the pair if one fails unless you are absolutely positive you have a matching replacement drive.