What happens if you DON'T copy the thread and post tables.
 

I'm planning on setting up a test mirror. I suppose this could be done faster if I can drop these two table out of the mirror (they'd still be in the mirror database, they'd just be empty). Can this cause problems? If so can they be avoided? Or am I better off biting the bullet and copying them too??

uh, isnt that the MOST IMPORTANT PART of a forum?

Second to the users maybe. :)

Then again, that is debatable.

I truncated almost everything for my test-setup - no problems (except wrong counters of course).

For the main boards, yes - of course. But a testing site doesn't have any users other than the admins. It's only purpose is to see if a hack will work correctly. It's a model, if you will, of the main boards.

you won't get real problems doing it that way,

but still a filled testboard isn't wrong either ;)