One of my Western Digital Caviar Black 1tb hard drives failed on me. It is part of a 2 drive mirrored storage array that I used to save everything worth saving, relying on the redundancy to save my bacon should equipment failure occur.
In this, it worked well. The existing drive had everything I saved, and Western Digital was fantastic, their RMA process is a joy to use. However, rebuilding the array caused me some troubles.
Once I got the new drive, I backed up what I could from the one good drive to another drive, just in case (good thing too). For some reason, I had taken the drive out of the array, which I think caused me this trouble. I installed the new drive, took note of which cable went where (so I thought), and proceeded to use the onboard RAID controller to create the new array. I think I may have chosen the wrong drive though when starting the rebuild, as now I have a shiny new 1tb of blank, unpartitioned space in my mirrored array. Thankfully what I lost wasn't terribly important, mostly game backups and other nicknacks, I saved the irreplaceable pictures on another drive as well as my Mac mini just to be safe.
However, I will be much more careful in the future should this happen again, and I will probably do a little more digging to see what the proper procedure is for replacing a drive in a mirrored array.