My new iMac is scheduled for Tuesday delivery. I haven't been able to use an external drive for Time Machine nor Super Duper backups. Ethernet Gigabit is working fine, using that to connect to the Airport. While this device won't really be as fast as USB2 (according to some reviews) it would be much cheaper than a motherboard replacement to regain the FW and even USB ports on this 'old' iMac. It could then be used upstairs for school work, etc.
A poor alternative is to create a new partition on the internal drive and put at least a SD backup there. I'm getting extremely paranoid about not having a current backup!!!
I'm hoping that Migration Assistant (which I've never ever used) will recognize and use the external drive with a 20 day old backup...