Ah, I encountered this problem too, some days ago on my opensuse machine. It is strange because it was a new problem which came without any software update and went away the next day. So I guess it was some hardware problem.
But try this: it is possible that your computer has many USB ports, some of them are USB 1 some are USB 2 (2 being the faster one). If you have attached your disk to USB 1 port, the speed will be too slow. (Actually on windows, it informs you that your disk can support USB 2 and is in USB 1 port bla bla)