Here's an old (9. juni 2005) Molebox support answer I got on this:
> The reason of this behavior is that MoleBox does not intercept
> system function 'CopyFileA()' used by Blitz Basic's CopyFile command.
>
> If necessary, you can copy the contents of a text file using a
> sequence of commands instead of single CopyFile command. You'll have
> to create an empty file, read the contents of the original file, and
> write the data to the new file.
>
> Please note that you may also need to use absolute file names in a
> Blitz Basic program which will be moleboxed. For example, instead of
> "original.txt" file name, you have to use (AppDir + "original.txt"),
> where AppDir is a string path for the application directory - I'm not
> sure for the syntax.
> system function 'CopyFileA()' used by Blitz Basic's CopyFile command.
>
> If necessary, you can copy the contents of a text file using a
> sequence of commands instead of single CopyFile command. You'll have
> to create an empty file, read the contents of the original file, and
> write the data to the new file.
>
> Please note that you may also need to use absolute file names in a
> Blitz Basic program which will be moleboxed. For example, instead of
> "original.txt" file name, you have to use (AppDir + "original.txt"),
> where AppDir is a string path for the application directory - I'm not
> sure for the syntax.