One file is a .xls file. It is either updated by a manual change, or by VBA code. There is also a second version of the .xls file that is created by VBA from the first .xls file. It is a copy with the date included in the name. The other file is a .dat file. It is normally updated by VBA in the .xls file by adding a record to the end.

I'm going to be updating it in a few minutes, and again in a few hours. It should backup every couple of hours. If it fails, I'll tate your advice and do the Diagnostics thing.