When I tried to mount a hard drive on FreeBSD today, I got the following error message.
sudo mount -t ufs /dev/da0p2 /mnt/ mount: /dev/da0p2: R/W mount of / denied. Filesystem is not clean - run fsck. Forced mount will invalidate journal contents: Operation not permitted
Here is how to fix it:
sudo mount -r -t ufs /dev/da0p2 /mnt/
where ufs refers to the file system of FreeBSD.