You can check if it is a problem with the power supplied by the SheevaPlug by attaching the disk via a powered USB 2.0 hub. This way the hub will supply the power to the disk. If it works better with the hub then the SheevaPlug is having trouble powering your disk.
Seems like a sensible check....
Mine's externally powered though, so it shouldn't be that. I have a couple of other enclosures I'll try to see if I get similar behaviour. Otherwise... nobody else has seen this then I take it?