I recently downloaded QB45 to a flash drive (FD), hoping to keep it from interacting with programs on other drives. I then wrote and saved a simple program to the FD, same directory as QB45. If I call this program (load file or open file) from within QB45 it runs perfectly. But if I double click on the BAS file in the directory, I get the error message "Bad file mode". It looks also like what's coming up is not QB45, but an old version of QB4.0 still on disk C. What can I do to get it to work properly?

It sounds like a file association error. Your BAS files are still being told to open with QB 4.0 but if you go to the Control Panel and click "Folder Options" you can change the ".BAS" extension to open with QB 4.5.

The reason that you're getting that "Bad file mode" error is because DOS applications sometimes have trouble interacting with removable drives and external storage. Plus if you don't follow the 8.3 filename format, that can definitely lead to problems.

If you have saved the .bas in binary mode the QB 4.0 may not be able to open it (hence the "bad file mode".

Just a thought.