How do I copy all photos from Photos App to an external drive?
Is your Lightroom installation also on your Mac OS partition?

If you want to edit the files in Lightroom without having to "extract" them into a separate folder, you should:


Open the folder containing the Photos library in Finder.
Right-click the Photos Library and click "show package contents".
Open the Masters folder and right-click on a year folder and select "make alias".  (This is necessary, since the "make alias" option doesn't exist when you right-click on the Masters folder for no apparent reason.)
Command+I on the new alias and click "choose destination", then drag the Masters folder into the dialogue that opens.
Rename your alias and drag it wherever is best for you.