I backed up my iMac HD today to a new external hard disk. It's a Western Digital My Book Personal Edition 160 GB USB 2. I know it's not the fanciest model but to back up 123 GB of data it took 3.5 hours. Is that normal? I used SuperDuper and it said it was averaging 10-11 MB per second transfer. I called Western Digital and the guy said if I wanted speed I should have gone with FireWire. He said USB 2 has bursts of 60 MB per second, but that 10 MB per second was pretty average. I thought it was 480 as a burst level? I kind of remember that USB 1 had a limit of 12 MB/sec....is it possible for some reason it was only running at USB 1 speeds? I'm using an iMac C2D which has USB 2 of course, and the cable I used I presume is USB 2 compatible as it came with the HD.