The Location of the Libraries only matters:
1) Not in the Cloud or on a Networked share. (must be local)
2) Must be in a specific location if you want PORTABLE to remember and use again. Portable can
access any Library that meets rule 1
3) Calibre (the installed Version) can use any rule 1 Library

Lock in (assign) the drive letter for
detachable drives (to a high letter: eg L: ) so Calibre will find it again from the Quick Switch list