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Where saying stay logged out so no data or files are written too while the import takes place, as the importer could be trying to access that file at the same time.

When you run the importer i'm sure you have to log in as admin,
its been about 6 months since i done mine and i'm begining
to forget

Actually reading that again I'm a bit confused:

Both boards will be turned off.
New threads board I'm importing into - stay logged in as admin?
Current classic board I'm importing from - logged in as admin or not?

If someone can clarify that as clear as possible it will help.
Sorry for such detail.