I've had similar errors on USB2 disks in HP ML350 G6's
This has always been fixed by either moving the usb cable to the rear usb ports or using a dual connector usb cable, the ones with additional usb plug for power.
Actually so have I on those machines. The drives would work briefly then fall back to USB1 speeds.
maybe trying one of these or similar would resolve the issue - not used them personally.
http://www.ebay.co.uk/itm/LINDY-1m-USB-3-0-Dual-Power-Cable-2-x- Type-Type-Male-/120988999684#vi-content
I'd forgotten all about those splitter cables... I wonder if I can parasite power from a USB2 port (I have a 4 port USB3 adapter with all 4 ports in use, although only one to a drive with no external power which is the one I'm having problems with) James