In simple, a USB/OTG cable is a Micro USB to female USB adapter that lets you connect USB enabled devices to your tablet/phone. You can buy this cable from $0.59 to $10.00 from anywhere and in Pakistan you can find it for around 100-1,000 rs. Essentially, you want to attach the micro USB end into your Nexus 7 and plug in a portable hard drive or flash drive (of any capacity) into the female USB port. But, storage permissions are not granted yet. Your device must be rooted and the bootloader must be unlocked in order for this to work. If you haven’t rooted check here. After this you want to download an application, STICKMOUNT, for free off of Google Play. Then connect your storage device and a Pop-up should appear. There you go, you can get unlimited storage data with these easy steps.
2. Wireless Hard Drive (No-Root) [Level: Beginner]
You can also use a Wireless Hard Drive such as this one here share data with your tablet (Android/iOS) OTA. Although, this method is pricey, I would recommend to keep your data safe and to make it a great alternative for extra storage on your Nexus 7. Here’s a video demonstrating the hard drive in action.