If you want to keep using Unity Hub for project management but save space on your main drive, use a symbolic link (symlink) to "trick" Windows.
If you're looking to work on your Unity projects from different locations:
2019.x (too slow asset pipeline for portable drives).
directly from your external drive's "Editor" directory without writing to the host computer's system files. Cross-Device Consistency
If you want to keep using Unity Hub for project management but save space on your main drive, use a symbolic link (symlink) to "trick" Windows.
If you're looking to work on your Unity projects from different locations: unity portable install top
2019.x (too slow asset pipeline for portable drives). If you want to keep using Unity Hub
directly from your external drive's "Editor" directory without writing to the host computer's system files. Cross-Device Consistency unity portable install top