Hi,
serious msdosfs bug has been recently found and fixed (see
e.g. kern/9116 and kern/9206 for possible symptoms). If you
had problems with msdosfs recently, update to fixed versions
of sys/msdosfs/ files:

$NetBSD: fat.h,v 1.14 2000/03/22 14:33:25 jdolecek Exp $
$NetBSD: msdosfs_lookup.c,v 1.47 2000/03/22 14:56:56 jdolecek Exp $

and try again, please. If there is still any remaining problem,
send-pr(1) it ASAP please.

Thanks,

Jaromir