The mystery of the Undead HDD. How can this happen?

The short version

I had a disk die on me a year ago. It had been a full-on click of death; the system couldn't see anything let alone mount it. A year later, I stumbled across the same disk, mountable and readable. How can this happen?

The Background

About a year ago, I had a horrible "Click of Death" from the boot disk in my Linux box (the disk also had nearly everything else on it apart from my VMs). I asked Hard Disk makes "Click, click, click ..." noise on system start. Won't boot at the time, seeking help to diagnose the issue, and checking if there was any way to get my data back. In fact, the answers were what taught me the term "Click of Death."

The outcome was I swapped in another disk from an unused workstation and rebuilt the system then restored what files I could from a recent backup. I also greatly strengthened my backup regieme and proceeded to go about my life / work.

As an afterthought I left the dead disk in the unused system I had grabbed its' replacement from. Not sure why, I just did.

Then history repeats itself

Nearly a year later, the same thing happened. This time, my better backup regieme means no important stuff has been lost.

Anyway I commence rebuilding the system, firstly grabbing a snapshot of my VMs from the other disk that's in there. During this time I had cause to fire up the other workstation I'd grabbed the now-dead disk from in the first place and booted from a live cd.

Here begins the mystery

Whilst checking the health of the unused system's second disk, I noticed that not only was the original dead disk powered up, but that Ubuntu had managed to mount it, and I could see all the files. This had not happened last year when I tried the exact same thing just after the disk failed. Naturally, I copied everything remotely important off it, and dropped it to a USB disk. The backups I restored from at the last rebuild meant my total lossage last year was minimal, but still I now have the luxury of looking for a few things I couldn't restore.

The upshot

In short I have a disk that died a year ago. It was dead, gone, kaput. No box I plugged it into could see that the disk was even there, let alone mount anything. Now, a year later it's back to life.

How is this even possible? And is this disk in any way trustworthy?

In short no the "dead disk" is not trustworthy. You may find that after some period of time and usage that the disk will die again, once the drive heats up.

It has been known for drives with the "click of death" to become recoverable after a period of time, or by using tricks like the freezer method. But in any case I would not use this drive as a system drive or save any important information on to it.