I appreciate your reply, but it doesn't address my question, which is how to mount a persistent filesystem from HD.

Some counterpoints:
1. Availability of CD/DVD boot is far more reliable than USB on older hardware. yes, I'm aware that I can clone knoppix to USB, then use a CD to boot and point to the USB for the root FS. I don't want to do that.

2. If I'm running a config within MS Virtual PC (and I do), I cannot use USB, period. I can however mount the ISO (from HD), so the speed difference between Knoppix on CD vs, Knoppix on HD is nominal (though I'm still taking a memory hit), therefore, no need to have the disc in the physical drive. Also, if I run multiple vms, I can use the same ISO image to host each of them - meaning FAR less disk usage. Dunno about you, but _I_ believe in backups, and as a result, I'm a bit picky about wasting storage space.

3. Raw speed is less of an issue as compared to needing to back up multiple copies of the data.

4. persistent fs overlay images allow for cloning stable configs - I can copy the data file for safekeeping, and easily "roll back" to it if necessary.

Mostly, I just need to update some CPAN stuff, and say, fprot or clamav to use a knoppix boot to check suspect files on a network share. Knoppix is not my primary work environment, so I'd just as soon not dedicate 8GB+ of my HD to it just to run a few tools, when half a gig would more than suffice.