Excerpts from netbsd: 12-Sep-97 Re: hang during shutdown maximum
entropy@zippy.be (1216)

> On an ffs filesystem that's already mounted
> I get:
> [...]
> ffs: /dev/sd0e on /usr: Device busy

That's impossible because for NFS one can't identify the
source reliably. A server could have more than 1 address.
It's unnecessary because the server will keep the
filesystem consistent.
In principle, an ffs device could be mounted more than
once, read-only, too (?) The fs code is a bit too save
here imo.

best regards
Matthias