Copying photos to USB Flashdrive
I have a PNY USB Attache flashdrive. All of my photos are in the Apple "Photos" application. I backed up some photos last year with this flashdrive and I still have a lot of space. I want to add photos but when I plug the flashdrive in, it goes to the photos I already backed up and there is no indication of how to add additional back-ups. I tried grouping the photos in an album in Photos and then exporting the photos to "Pictures" under documents in the finder. I tried dragging and dropping the photos from Finder but it doesn't accept them. Any ideas?
iMac, OS X El Capitan (10.11.3)