I have a patch for the crash, but it shows and arguable regression in 
bevavior against 21.4.19.

Old behavior of f-n-d was to return non-existent drive unmodified.

New behavior rerturns nil in such cases.

docstring says to return nil if FILENAME does not include a directory.

This leaves some room for mis-interpretation.

Consequentially C-x d f : RET will wipe that entry on completion, where 
old behavior left it intact and said [No match] after it.

I'd appreciate your opinions.

Regards,
Adrian

With my fix (courtesy Mozilla project) :

(file-name-directory "c:")
"C:\\Programme\\XEmacs\\XEmacs-21.5-b29\\i586-pc-win32\\"
(file-name-directory "f:")
nil
(file-name-directory "NUL")
nil
(file-name-directory "/")
"/"
(file-name-directory "\\")
"\\"

21.4.19

(file-name-directory "c:")
"C:\\Hacking\\cvs.xemacs.org\\XEmacs\\xemacs-21.4\\src\\"
(file-name-directory "f:")
"f:"
(file-name-directory "NUL")
nil
(file-name-directory "/")
"/"
(file-name-directory "\\")
"\\"