Hard drive problems

I have a SimpleTech hard drive that I use for back up. I was copying my music from my old Toshiba on to my MacBook Pro when I get a pop-up message saying that the hard drive wasn't ejected safely. I didn't eject it and it was still plugged in, but it stopped showing up anywhere on the computer. So the copying obviously stopped and I had to do it again, but the same thing kept on happening after about 7 minutes. That's happened about five times!
Help?

It appears your SimpleTech might be failing. Get what data you can from it (preferably everything) as soon as you can and then use Disk Utility to verify and repair the drive.

Post back with the results.

Regards.