how can I add photos from an external drive to the new Photos I have a large library of old photos stored on my External Hard Drive (EHD). I would like t bring these old photos into Photos (Mac OS X Yosemite). How can I do this, what's the best way to do this?
Open the Photos app, go File -> Import, and navigate to the folder with the photos.

If the photos can't be imported using the Import feature in Photos and are in an iPhoto library, you could first open the library with iPhoto, go File -> Export, and export the originals. They could then be imported using the Photos app.