Oddness attempting to mount ext2fs

[Justin R. Smith]

My uname:

FreeBSD jsmith.org 6.0-BETA3 FreeBSD 6.0-BETA3 #0: Tue Aug 23 08:04:37
EDT 2005     jsmith at jsmith.org:/usr/obj/usr/src/sys/MYKERNEL  i386

The command

mount_ext2fs /dev/ad5s1 /linux

results in the message

mount_ext2fs: /dev/ad5s1: Operation not permitted

and in dmesg, the message

WARNING: R/W mount denied.  Filesystem is not clean - run fsck

Unfortunately, attempting to mount it read-only doesn't change anything :-(

Any suggestions?\

(Incidentally, the linux partition in question was clean, at least
according to Linux).