I don't have much use for .NET, but since no on else has responded... I've tried this code non-DLL (~5 minute using VS2010 Express is all I can tolerate
) and it works fine. Have you done so, as well?
I just want to be sure that this issue is only related to the DLL or how it is called (not e.g. a D:\ root permissions/FAT32 issue). That being the case, show us how it is called (and make sure it is called only once). Otherwise, make the files/file references in a folder and see if same result.
Also, couldn't help but notice the 'xlTextMSDOS'... which seems odd. Is that intentional (as opposed to 'xlCSV' or 'xlCSVMSDOS')?
Use of a mobile device to post here is problematic, at best, and perhaps negligent, at worst. I'll try to keep it to a minimum.