Its plugged into the back slots, no other usb device is connected, but there was
a very non-compliant usb cable in another slot. The USB HDD is a new usb2
Seagate 1TB ST310005EXD101-RK

At the moment the thing's still doing work. Once its done, which will take days,
I'll add a USB2 card and try it in that. Then I'll know more.