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TheArtfulDodger - Calibre Portable for Windows is designed to be
self contained on a portable storage device -- i.e. something you can put in your pocket and use on another computer. I don't think calibre-portable.exe accepts any arguments to pass onto calibre.exe, I just tried
--help and
--start-in-tray without success.
You could do as
ilovejedd suggests, use a junction point to the external library folder, but doing that will subvert the purpose of Calibre Portable. You could overcome that by using a symbolic link referencing a relative path; a junction point can only reference an absolute path.
However, you wrote the library in question, "My Main Calibre Library", is on the same storage device as the "Calibre Portable" folder - so why not move the "My Main Calibre Library" folder into the "Calibre Portable" folder - KISS.
IIRC calibre portable will start with the last used library.
FWIW: ==>>
windows - What is the difference between NTFS Junction Points and Symbolic Links? - Stack Overflow. It's the best comparison of the two I've come across.
BR