Hi there,
I use Knoppix 3.2 and 3.3 and have the same problem. I try to save my harddisk to an USB-storage Maxtor 500LE with the following command:
dd if=/dev/hda of=/mnt/sda1/hda.img
But after a short time the system hang totally. The only way is to switch off the power supply.

But i got a Knoppix 3.0 and this old Version works fine.
I estimate the parallel support from UHCI and OHCI since KNOPPIX 3.1 reported in ftp://ftp.leo.org/pub/knoppix/KNOPPIX-CHANGELOG.txt is the reason.