My 500GB WD external HDD was working fine a few months ago, and I've just plugged it in again. It is now unrecognised by my OSX, Ubuntu and Windows boxes, and doesn't seem to spin up when connected. It makes a quiet, repetitive humming that comes on and off about once every second.
Any ideas/suggestions greatly appreciated, however blunt (I don't think there's anything I'd cry over on it).
Fixed, with help of @RJFalconer — mounted the drive in a new case. It is interesting to note that the WD caddy actually used the legacy power connector to the drive, while the new case uses the SATA power cables. I don't know enough about drive to know what exactly was happening, so I'll just be thankful it's fixed.