USB Media Viewer, formerly known as Nexus Photo Viewer, is a free Android app that lets you view JPEG and RAW photos from a USB flash drive or SD card on your Nexus, Pixel, or other Android 4.0+ devices supporting USB Host.

To use the app, you need a USB OTG cable that costs approximately $1-10 USD on Amazon, a USB flash drive, or a USB SD card reader and SD card. The app does not require root access and can be used without technical knowledge.

It is essential to note that the Nexus 4 does not support OTG/USB Host and cannot be supported. The app strongly recommends external power when connecting a hard drive to a tablet. For more information, please see the troubleshooting guide on the developer's website. In addition, it is advised not to connect an unpowered hard drive to a phone.

Overall, USB Media Viewer is an easy-to-use app that provides a convenient way to view photos on Android devices. The app's technical notes and how-to videos make it easy for users to set up and use.