I see what you're asking. All you need is a cable like this.
Amazon.com: IVSO? Nexus 7 Tablet Micro USB Host OTG Cable - Micro USB B/Male to USB2.0 A/Female OTG Host Cable for Nexus 7 (Black): Computers & Accessories
It doesn't have to be that exact brand just USB OTG. I actually bought one at tiger direct (retail) the other day and it works great. When i was in Atlanta I bought two different kinds. One I bought was junk and diddn't work. Was too lazy to return it. The other I bought worked great for a while but the inside wire must have broke.
Fortunately they are cheap cables $5-$10 max. Just read reviews on which work the best for people. All you do is plug the "USB" side into the USB OTG adapter. Regardless what the other side is micro usb or USB 3.0 it will work. You just need the magic usb OTG.
Someone said power adapter so what they mean is just plugging the charger source into the External Hard Drive and connecting the hard drive normally as you would to the nexus 7 using the OTG cable. However I do have a Passport and (forgot the other brand) but one is 1TB and the other is 500 GB. The TB is USB 3.0 and I don't ever use an external charger. Same as the other. It might be bad for it but oh well.
You might have to be rooted to use an OTG cable. I think android central made a guide a while back explaining the process for rooted and unrooted. I'm rooted and just used stick mount but I also have media importer. And I will say with my 32GB USB drive I feel like I'm using a micro sd card. Roms, movies, music, everything loads off it perfectly and can be copied. I might get a bigger USB drive or the meenova. But I can literally store countless roms, music, and movies onto it and I can't save the internal space for games and apps on the nexus. It makes the lack of a micro sd card slot almost pointless.
Best of luck.