
When you attach a device, Photos scans its contents and compares them to its current library.

When you attach a device that your Mac registers as one containing photos and/or videos, the Import pane (or Import section of the sidebar) appears, ready to gobble your media files. If you’re shooting photos (or capturing video) with a standalone camera, or with an iOS device that isn’t syncing with iCloud, you’ll get your files into Photos by attaching the device or its memory card to your Mac. Choose Image Capture from the Connecting This Device Opens menu. To make Image Capture the default app for a device or memory card, open Image Capture, select the card or device under Devices, and click the disclosure triangle at the bottom left. Once the files are imported, you can modify and arrange them, and then add them to Photos using the File > Import command in Photos.

The Image Capture app is a great alternative to importing your files directly into Photos.
