There are two ways to 'Import' photos:
- from camera or card reader
- from files and folders.
In the first case, you are supposed to want to copy the files (import) from 'external media' to your computer drives (internal or external) then to 'register' or 'index' the image files in a catalog.
In the second case (which includes importing from USB external drive), you are supposed to keep the files where they are and only to register/index the files in the catalog.
There is no difference between internal or external drives. You don't copy anything. You register the current location of files (drive identification and folder path) in the catalog.
External media like cards, CD, DVD let you choose an option to copy or not and if you choose not, you can create special thumbnails to manage the data in your catalog without having the media plugged in. If the external media is not plugged in, the files are shown as 'offline' (not 'disconnected'). You can manage the catalog (add tags or captions, rate) but you can't edit them obviously. That was very current at a time CDs were the only external media with photo cards.
There is no way to use the 'import' button to copy the files from an external drive. You can copy the files with the Explorer/Finder before using the import button. Or once the files are registered in the catalog, the catalog manager can move the files anywhere in other drives.
What is special is using USB thumbsticks: they can be considered either way, 'from files and folders' or 'from camera or card reader. Your choice.