My sheevaplug has suffered the same fate, and I have NEVER used an unpowered usb hard drive on it. It had been running 24x7 with an unpowered USB hub + 3 powered usb hard drives.
3 people I know bought sheevaplugs at the same time and 2 of their power supplies died within a week of each other. So don't be surprised if yours also goes pop...
On a positive note it looks like there is currently a 100% success rate at using a replacement power supply to get the plugs working again.