If the external drive is NTFS formatted, I would avoid writing to it, there is a chance that Knoppix could corrupt it beyond recovery. I prefer to salvage files by transferring them to another computer on a network. However, if the external drive is formatted with some flavor of the fat format file system you can write to it just fine. You may need to mount it read/write rather than read only. See the man page for mount for details (type man mount at a console prompt).