Theoretically you *should* be able to get SID from a Knoppix install but due to the mixture of packages (i.e. stable/testing/unstable) it is sonetimes difficult to get it to work as one would expect. Also those who DO isntall Knoppix to hdd seem to have no end of problems recompiling the kernel. I would suggest using Kanotix for an hdd install as it was designed for just this purpose (and it is already built from SID) No kernel problems, no apt-get upgrade problems. It just works.

BTW Knoppix was never meant to be installed to hdd.