I have had several 500GB external drives fail, and all of them were able to continue working after I removed them from their enclosures and put them into new enclosures. In each case the electronics of the interface had failed.
My current problem is with a Seagate 500gb Barracuda internal hard drive that came with my Dell XPS 730 computer I bought in November, 2008. There had been no signs of drive failure (noises, messages, clicks, etc). One day last week I shut down the computer. When I turned everything on later that day, I got a message indicating there was no boot device. Going into the BIOS setup I saw that the computer was not recognizing the presence of the drive. Dell sent a replacement drive, but there was data on the original drive that was a victim to "good intentions to back it up this week." My bad, of course.
Ironically, I received a letter and CD in the mail from Dell that same day with a firmware/driver upgrade from Seagate, indicating that there was an "industry-wide" problem with that drive, and the upgrade would prevent the problem. Four days too late for me.
I tried putting the drive in an external enclosure, SATA-USB, but my other computer would not recognize it. It is now in the hands of the Geek Squad at Best Buy, on the advice of several friends who had good results with data recovery from them at a decent price.
Question: several people in this thread have mentioned rescue software, but how would that work if the computer cannot find the drive?