powered down drive still manages to fail

I installed a primary disc as a slave disc in order to recover data from it.

It was already failing at this point, I believe, due to a brief power spike.

Eventually I powered it down, leaving it in the desktop.

Things were fine for a while. Then, I started hearing clicking sounds coming from the slave disc. Disconnecting it stopped the noise but no boot-up.

I had to install another disc in its place (or re-boot from CD).

How can a drive fail if it is powered down?

Does powering it down still leave it running in some way?

This is the puzzle for me.