in addition, you might check to make sure the usb drive is not formated in ntfs instead of fat32. also, the checkmarks are not enough to make the drive writable. (depending on your release version) you need to right click on the drive and select 'actions' and 'make drive read write' <- or something similar.

before you try copying critical files, create a small text file (call it 'test' or some such) with one of the many text editors provided in knoppix and see if you can save it to your thumb drive. if you can, then you will be able to write the files you are attempting to salvage.

not too sure why you might be getting the 'can't read file' message, but maybe the above information will give you a little more to go on.

there's probably a slick shell command that will allow the drive to be mounted as writable, but i don't know if your standard "mount mnt/dev/" string will do that or not. have to try that myself next time i drop into knoppix.