This seems to be a problem with a new kernel extension in SL
There's a few discussions on teh Interwebz about this issue which is not apparently confined to the use of SuperDuper!
http://discussions.apple.com/thread....rt=45&tstart=0
http://www.xlr8yourmac.com/
among others...
Googling "Snow Leopard Slow Firewire" will find more.
A workaround is to replace a SL .kext(s) with the one(s) from 10.5.8
I did this and the copy that took 12 hours
last night seems to be going a lot faster today, should finish in less than an hour or so.
Note that messing about with your system files can cause unintended consequences and if you aren't sure what a kernel extension might be then perhaps you should wait for an official fix...
