Hard disk are about one of the more power hungery components of any computer system.  This isn’t normally an issue when the hard disk is inside your computer where it can pillage as much electricity as your power supply can manage.  But take a hard disk outside your computer and the story is different.  Think along the lines of a USB external hard disk, with a power supply of course.

Now I use one of these as part of my backup system.  Not perfect (what backup system is), but for the most part it works.  That is until the electronics inside the enclosure decide not to provide enough power to the disk.  Take the disk out of the enclosure, plug it into your computer and you can hear it spin up when the pc is turned on.  You can even read data from it so you know it works.   Put it back into the enclosure and turn it on.  What do you hear?  A little ticking noise as the disk tries to power up and spin but doen’t have enough power.

Happened to me twice now. Two separate disk, two separate enclosures, two different power supplies.