Similar issue here: just bought a 2TB CnMemory Spaceloop USB 3.0 HDD (Western Digital WD20EARS:http://www.storagereview.com/western_digital_caviar_green_2tb_review_wd20ears
As "just" 5400 RPM hoped for a faster spin-up from power save mode as with my current Hitachi 7200 RPM drive. Cloning (rsync) the content both USB-drives connected via an USB-Hub to the SheevaPlug's USB-Port worked fine.
However why I tried to attach the 3.0 HDD directly to my SheevaPlug it's not recognized.
Same result with a friends SheevaPlug (he also bought that CnMemory). Though his GuruPlug "sees" it.
Seems I've to stick with an USB-Hub as "converter", now. :-(