Hi c_cinq,

KNOPPIX is very good (sorry not perfect) at hardware
detection. A usb CDRW should work just fine when booting
from the KNOPPIX CD. It will be automatically mounted
as /dev/cdrom1 /mnt/cdrom1 if KNOPPIX is on /cdrom.

I find a second CDRW (perhaps a DVDRW is in my future )
to be an excellent addition to a KNOPPIX system.

Yes K3b will work just fine -- although you can easily learn
the shell commands for cdrecord to blank and write to it
directly.

Two cautionary notes are in order:
1) packet writing is not yet here for KNOPPIX -- now you can only
blank a whole CDRW and the burn to the whole CDRW.
2) With a hard drive install it will take a little more work to get
the usb drive mounted. One way of course is to get the usb
CD working from the CD boot and then reinstall. To make
a hard drive install work without reinstalling you have to things
to /etc/fstab and I am not the one to ask about that.

My project for automatically saving and restoring /home/knoppix
to a CDRW is very preliminary -- but here is a link for what it's
worth.
http://www.knoppix.net/docs/index.php/BurnHome

Best Wishes,
paradocs