1. If you will move files between directories; that means you will delete
them and recreate in new location.
2. Probably you use FindFirst, FindNext (procedures names from Delphi, in VB
names can vary) to find file to process.
3. The directory contents will be changing when you will delete files and
create new ones.
4. That'w why FindNext will be work improperly

You should make a list (collection) files to process before any directory's
modification.

--
Tomek R.