Thanks chuckc. Jeffrey was right that Samba is renaming the file, but no amount of reconfiguring Samba will allow you to use a colon in a filename in Windows (just try creating a new text file with a colon in it on your Windows box). The only solution is to add it to BAD_CHARS (which I have done). Waiting for the next airing of The Exorcist to confirm that it works as desired.