I will address the sd card question and leave the external hard drive questions, to many of our users who have more experience with them.
To move photos to and from the Nexus 7 you will need and OTG cable and the Nexus Media Importer app, which I now understand supports export from the Nexus 7 to your SD Card. Personally, I have a Rooted Nexus 7 and use the app, Stickmount.
I will say that Nexus Media Importer, recommends a power source for an external hard drive.
I am getting the ball rolling on your issues, expect lots of input from our members.
Sent from my Nexus 7 using Tapatalk 4 Beta