Problems (from most annoying to least annoying):
(1) unmounting will restore the mount point as a directory, but
further mounts on that mount point will fail, as it's still busy.
(2) when asked to mount such a file system as xenix (or sysv), the
mount point magically becomes a useless file.
(3) there's no direct way to mount such a file system (have to copy it
somewhere else, without that first track, and mount it from there).
# ls -fl /mnt
total 2
drwxr-xr-x 2 root root 1024 Aug 4 1995 .
drwxr-xr-x 24 root root 1024 Apr 3 17:34 ..
# mount /dev/fd0 /mnt
mount: block device /dev/fd0 is write-protected, mounting read-only
# ls /mnt
# umount /dev/fd0
# mount -t xenix /dev/fd0 /mnt
mount: block device /dev/fd0 is write-protected, mounting read-only
# ls -l /mnt
?--------- 0 root root 0 Dec 31 1969 /mnt
# cd /mnt
sh: /mnt: Not a directory
# umount /dev/fd0
# ls -fl /mnt
total 2
drwxr-xr-x 2 root root 1024 Aug 4 1995 .
drwxr-xr-x 24 root root 1024 Apr 3 17:34 ..
# mount /dev/fd0 /mnt
mount: block device /dev/fd0 is write-protected, mounting read-only
mount: /dev/fd0 already mounted or /mnt busy
# dd </dev/fd0 >/tmp/fd0 bs=18k skip=1
79+0 records in
79+0 records out
# ls -l /tmp/fd0
-rw-r--r-- 1 root root 1456128 Apr 13 21:18 /tmp/fd0
# ls -fl /floppy
total 2
drwxrwsr-x 2 rdm floppy 1024 Aug 4 1995 .
drwxr-xr-x 24 root root 1024 Apr 3 17:34 ..
# mount -o loop -t xenix /tmp/fd0 /floppy
# ls -l /floppy
total 2
drwxrwxrwx 2 root daemon 64 Aug 30 1992 install
drwxrwxrwx 5 root daemon 80 Aug 30 1992 new
-- Raul