Install Mint/Ubuntu to a external harddrive or flash drive.
Channel: USBTux
Date: 2013-05-20

*When using with windows 8 (UEFI-based systems) or above you need to use a boot loader - I've had good results with rEFInd boot manager.*

 This is for Mint 16 or below, or any Ubuntu derivative using the older installer
Following the screencast  to install to any harddrive, internal or external or even a flash drive, it just gives specifics for external. Obviously just change the locations as required, special notice should be made to "Device for boot loader installation:" We want to ensure the boot loader is written  to the correct disk.
With the boot loader on the usb disk it will not interfere with the internal disks at all.