If you connect your drive to the computer you get the drive letter of the first free letter. that can differ between the systems you use. Some people want/need a fixed drive letter for their portable drive (thread). Drive B: is normally unused on most computer. So the idea of some people was simple: Mount the portable drive to a virtual drive. Windows itself got all tools (normally) onboard. It is done by subst.exe.
This small app mount your portable drive virtual to drive B: (no selection possible).
cu & hf
ps: thanks to MarkoMLM for help