From what I understand, there are 3 apps worth looking at. There's Stickmount, which was the initial app that kind of got this ball rolling. It works great but I had some issues with it. Nothing major, but even when my disk was ejected I could still see the data in the directory. It was weird.
There's also Nexus Media Importer, but it's read only - however it doesn't require root. I don't know about you guys, but 99% of my OTG uses come from copying to the device only, so this would work fine for me. Typically this is how I handle this... I'll have my 500GB 2.5" WD HDD (yes, it powers itself entirely from the N7 port, so long as I use a short/thicker USB cable) with tons of movies, shows, concerts, etc. I'll copy what I want to the N7 and when I'm done, I delete it. Then copy more stuff over, watch it, delete, etc. It's not a bad gig. Some people get in a tiffle over the fact that the N7 doesn't have MicroSD, but even if it did, unless they made super super affordable 350GB+ MicroSD cards, I'm not sure it'd matter. It's very easy to just copy stuff over in a few minutes, eject, and be done with it.
There's also a third app, which is what I use. I don't believe it's in the play store... I found it on XDA. It's called USB OTG Helper and it does require root. My drive is formatted as NTFS (which makes no sense that these apps wouldn't support EXT2/3/4, but whatever) and works flawlessly with read/write in both directions.
[APP][ROOT] USB OTG Helper (Stock/Custom kernel) - xda-developers
For those of you having USB OTG trouble, first thing to do is swap out the cable. I've seen time and time again (even first hand with my own tablet) that perfectly functioning cables won't work with USB OTG, but once I picked up a short/thicker cable, things worked great. My 2c.