I wanted to share some of my findings about USB 2.0 and Time Machine on my iMac. I have a USB Port Expander made by Belkin and had a USB 500gb hard drive going through the expander. I began to notice how poorly time machine was running and how it would keep the hard drive writing all of the time. At the same time I noticed that my IPOD and IPHONE, also connected through the USB expander, was taking forever to update and sometimes failing. I did some testing and found that the USB 2.0 hard drive works best plugged directly in back of the IMac. If you use it through an expander it WILL not work as well. The port expander make/brand does not seem to matter much, as discovered upon further testing. Time Machine is ok, but I am not impressed as much as I would like to be. It takes too much storage space and I can't really see any compression schemes in use, which would be very helpful. I am not sure if it is really doing an incremental backup either. Conclusion. I would recommend anyone who uses time machine to get a hard drive that is at least 2x the size of the one they are backing up and to get Firewire 400/800. USB 2.0 does work, but is not as reliable as FW and is very picky when sharing the bus with other devices. I hope this helps someone. Thanks.