error when mounting md0 invalid argument

[David Mehler]

Hello,

I've got a copy of the FreeBSD 12.0 memstick img file. I'm trying to
mount it to make some changes to allow it to go serial terminal by
default. I'm doing:

mdconfig -a -t vnode -f path/to/img
and have also added the -u0 option to that line as well.

An mdconfig -l does indeed show /dev/md0 and file -s /dev/md0 shows
dos/mbr imgage  I believe that was. I then try:

mount /dev/md0 /mnt

and get invalid argument. Nothing in the logs. I've tried
mount_msdosfs and mount_ufs same result invalid argument.

Any suggestions appreciated.

Thanks.
Dave.