There are two routes to connecting USB external magnetic media like pendrives and hard disks: unrooted using (NotFree) Nexus Media Importer app which gives a very limited read only capability, rooted with free Stickmount app which gives more or less complete read/write access and integration. The Nexus 7 currently can NOT access USB hardwired external media without a helper app.
The USB hardwired connection must be via an usb-OTG cable or adapter. WiFi and Bluetooth access is also possible, BTW.
-- nexus media importer and USB devices seem to work best if ALREADY connected BEFORE booting your N7.in my experience.
-- some hard drives are not recognized by NMI, some are - that's just the way it is. Someone with Stickmount experience can let us know about their experience.