Q: Import photos from other drive

You could create a second Photos Library on the external drive.

You switch between libraries by double clicking them to open in Photos. Then you can use Photos to browse alternately your main Photos library on the internal drive or the additional photos on the external drive.

See this Help page:

https://help.apple.com/photos/mac/1.2/?lang=en#/pht6d60b524

one way:

create a new fotolib with "alt" - Foto

in Fotos "command ," - that is "Einstellungen" 

deselect "Importieren" if selected

NO, external drive  only FAT XXX

A Photos Library should be on a drive with the file system MacOS Extended (Journaled).  Other file systems can cause problems with conflicting filenames.

Ok, that's the "fine" way. On the other hand, if you have a 3TB external drive and would like to use it either on a Mac or on a PC then you need to have at least FAT (as far as I know, or what about Sierra, can it write on NTFS?)

(as far as I know, or what about Sierra, can it write on NTFS?)

NTFS is read-only in macOS Sierra, there are some fixes to enforce it using the Terminal, but I never tried it.

But even read-and-write access would not guarantee that a Photos Library will work there.

If you want to use a large drive both with a Mac and Windows I'd partition the drive. A small partition for the Photos Library, and exFAT for the Windows partition, so the Mac can write as well.

You can store files and folders on a shared partition, just not a Photos Library.