Apple has posted Knowledge Base article #25532, explaining what to do if the alert, "Error in user parameter list (-50)" appears when working with an MS-DOS-formatted volume.
"Certain characters that can be used in a Mac OS X filename are not allowed in MS-DOS filenames. This can prevent the file from being copied, and can occur when a filename contains non-printing ACSII characters, or any of these characters: ? / " | > * [Ed. - See today's "Troubleshooting Acrobat Distiller 6.0 report" for another instance of filename character incompatibility.]
"Remove these characters from the filenames before copying.
"Alternatively, for a longer-term solution, reformat the volume as Mac OS Extended (HFS Plus). Be sure that any important data on the volume has been backed up first (after renaming any files with characters mentioned above), and that any other computer you will use the volume with works with Mac OS Extended format disks.
Apple adds that if the alert appears when you try to change a file, try copying it to the computer's hard disk first, then copying it back when you are finished.