We prepared bootable images with Ubuntu and ROS preinstalled. They support 64-bit architecture and both UEFI and Legacy boot. The images can be found here. Choose either Ubuntu 16.04 with ROS Kinetic or Ubuntu 18.04 with ROS Melodic.
You will need an USB drive with at least 8GB storage capacity. To flash the image you can follow the same instructions as when flashing Rover image to SD card:
Keep in mind that this will wipe all your data from the drive.
Now boot your computer from USB drive. Choose the first option in GRUB menu. User should be automatically logged and desktop environment should appear.
Ubuntu system password: ubuntu
This way your root storage will be extended to full storage capacity of USB drive - by default it's limited and would end up filling fast will any additional software installed.
Open terminal (Ctrl+Shift+t shortcut) and type: