I have an unbound text box that has a calculation in it. I need to be able to bound this to a field in another table, but it wont allow me to. Is there a way of doing this?

You can't bind controls on a single form/report to multiple tables. All controls are either bound to a column from the form/report's recordsource or they are unbound. You can use a query that joins multiple tables as the recordsource of the form/report and that would effectively allow you to have bound columns from different tables.

The short answer is if you want to update a column that is not in the form's recordsource, you'll need to do it by running an update query from code or writing an ADO/DAO update routine.