^ Those are you best bets, transferring HDD to HDD over SATA you will get rates of 140 Mb's per second.
I take it your trying to back up all your steam folder to USB2 external hard drive? For future reference you really ought to invest in ESATA. Over ESATA you can transfer 5Gb every minute so that would take about half an hour to back up your whole steam folder. Why ESATA? ESATA is a lot faster for copying over lots of smaller files then USB3.