mount failure on small NetBSD

[fab <fab%gnux.info@localhost>]

Hi!
I just (less or more) finished a small netbsd (fits uncompressed on some
8M CF) with mplayer and mpc/mpd. It's designed to be independent music
player with web and mpd interface. Actually music can be played from
network but I can't mount anything. When I plug an usb key I get this:

mount_msdos: /dev/sd0e on /mnt: Operation not supported by device

I ktrace'd the mount_msdos but can't find why it fails, the mount() call
fails but I can't understand why. If someone have an idea...

Regards.
Fabien Devaux.

PS: maybe it's important: the system is running with a trick, /etc/rc
exec() /bin/ksh