One is that the enclosure has failed and the drive is fine. You put the drive into a new enclosure and it springs back to life.
The second is where it doesn't come back and to kill the data, the sledge hammer does the final erasure.
I have an eight year old 500GB external drive that no longer mounts on the host computer desktop. Putting my ear to the enclosure, I hear the rhythmic click, click sound that tells me it is not functioning. I was using this drive as a Time Machine backup on my second (seldom used) Mac. My question is, since I was using it as a Time Machine backup, it therefore contains financial and other personal data that I need to "wipe" before I trash it, but how can I do that when the drive won't mount? Should I just drill some holes through it, or give it a good smash with a sledge hammer? I would appreciate any ideas. Thanks.