You can find your USB device when you go to ‘Launcher’ → ‘Files’ → ‘USB’.
When you connect a USB device to your Chromebook, the files have to be compatible with Chrome OS in order for your Chromebook to show them. When the files are not compatible, they will not show in the folder of your USB device.
*Important! When you want to remove a plugged in USB drive from your Chromebook, Click on Eject 
, this way you make sure no data will get damaged.