Copy photos from an iPhone to a USB flash drive I can use Photos.app to import photos to my MacBook's internal SSD, but I can't figure out a way to copy them directly to my flash drive. 

Is there any way to copy the photos without doing something insane like paying for some closed-source file-copy app or copying one batch at a time to avoid filling up my SSD?
I would suggest using Image Capture.app. It is located in your Applications folder. From there you can select your iPhone and import the pictures.