One of the black rubber bands has broken, and must have dropped (!!!) the hard drive from near the top of the drive cage to the floor.
My computer still works fine for the moment, but I hope it hasn't caused any long term damage to the drive.
Needless to say, I'll be putting the hard drive back in on a tray instead.
Update: I just had a closer look, and it turns out BOTH rubber bands broke. Now the hard drive makes an ominous click noise when you turn it upside down, and I doubt any damage would be covered by warranty
