A few months ago I consolidated five small drives of referenced files to one 2TB drive. I used Aperture to relocate the masters because I thought I'd make a mess of my library if I used the Finder. I was satisfied that I had moved all my images, and I wiped the drives. I wiped my backup too so I could back up the new drive.
Recently, while looking for something else, I realized I'd missed a couple of years old projects- one of them a documentary project. Fortunately, I had not updated my offsite back up yet, and I was able to retrieve the files.
This story has at least three morals:
1. ALWAYS have at least two backups.
2. Periodically- and especially after major file structure changes- go to all photos and filter for offline images to see what might be offline so you can fix it before altering backups.
3. User error can happen to anyone.
End of sermon
Pro, Mac OS X (10.7.4), 2.2GHz quad i7/8GB RAM/1GB VRAM