Doesn't writing the file directly to the output directory work?.. What's the point of writing the file to the "%TEMP%" ("C:\Users\%USERNAME%\AppData\Local\Temp") first then...
Earlier I was joining some gargantuan files using Bandicut and it just failed without notice (due to insufficient disk space on "C:\").
The operation seemed to be successfully completed without error, while the output file ends in truncation.
Note: I use "High-Speed mode" only.