> mount -o ro /dev/wd0a /

I would use

	mount -u -r /

Keep in mind that anything that wants to write
to the root fs (passwd, /tmp, ...) will be unhappy.

> sync
> fsck -fy /

fsck -y is dangerous.  I would use

	fsck -fp /

Hmmm, I just tried this and fsck complained:

	NO WRITE ACCESS
	/dev/rsd2a: UNEXPECTED INCONSISTENCY; RUN fsck_ffs MANUALLY.

Also tried /usr (no possibility of unreferenced files in /tmp)
and got the same thing.

Wierd.  Seems like it used to be possible to fsck filesystems that
were mounted read-only.  Perhaps this changed with the invention of the
"fs is clean" flag?  I don't see an option for not setting the flag.

Fsck also refuses to check a fs mounted read/write.  (I happened
to have an empty fs handy, so wasn't worried about losing any data.)
Leaving off the p flag didn't help.

Looks like you will have to skip the fsck.  Or run fsck on the
unmounted copy after you dd it.