I want to use a knoppix feature (bootfrom cheatcode) requiring a copy of the knoppix CD in the form of a regular file. (Placing that file on the hard disk lets it pinch hit for the CD itself next time you boot so the CD drive gets freed up.) The way to make such a file is with the dd command (dd if=<cd device> of=<filename>).

What if the only available platform for running dd is knoppix itself? With knoppix running, the CD is mounted and you can't unmount it. It goes against my instincts to copy a mounted filesystem. But I seem to have gotten away with it. And being a CD, it's nonvolative, maybe that's why. The resulting file ("knoppix37.iso") is useable with bootfrom ("knoppix bootfrom=/dev/hda1/knoppix37.iso"). System come up just fine and I'm free to take the knoppix CD out of the drive. I do notice a slight discrepancy in size of the file, versus one I made from an unmounted copy of the CD on another machine (400 bytes out of 700 gig). But it's all functional.

Any perspectives or downside warnings against dd'ing mounted filesystems, here or in general? Or should I stop worrying about it?