fsck doesn't recognize FreeBSD filesystems (9.2)

[Christoph P.U. Kukulies]

I just was wondering why the following did not work:

root at isdn:/dev # mount /dev/ad6s1a /mnt/ad6s1a
mount: /dev/ad6s1a: R/W mount of / denied. Filesystem is not clean - run 
fsck.: Operation not permitted
root at isdn:/dev # fsck /dev/ad6s1a
fsck: Could not determine filesystem type <<<<<<<<<<<<<<<<<<<<<<<<
root at isdn:/dev # fsck -t ufs /dev/ad6s1a
** /dev/ad6s1a
** Last Mounted on /
** Phase 1 - Check Blocks and Sizes
** Phase 2 - Check Pathnames
** Phase 3 - Check Connectivity
** Phase 4 - Check Reference Counts
** Phase 5 - Check Cyl groups
1867 files, 82726 used, 423761 free (3385 frags, 52547 blocks, 0.7% 
fragmentation)

***** FILE SYSTEM MARKED CLEAN *****


I had thought that FreeBSD could recognize its own filesystems (though 
this one is from an earlier version of FreeBSD)

--
Christoph