This question already has an answer here:
- Install on usb external hard drive 2 answers
Is it possible to boot ubuntu from an external Hard drive so that I can carry around a hard drive and plug it into any computer and be able to boot up Ubuntu?
K here's exactly how to do this.
1 Plug hard-drive into a computer with an OS (Duh)
2 For windows: Download universal usb installer 2 For linux: Get linux pen drive installer
3 Go into the installers (Make sure you have the linux OS you want to install)
4 Format the drive as you would with USB stick (In other words, pick the external hard drive letter in the menu and click continue pretty much)
The installation should enable you to run ubuntu from the external itself the way it can run from a USB
If this doesn't make the OS exactly as it would on a computer than run the OS and get a partitioner, leave 5gb for your current OS on it, than format the rest of the drive as a fat32 and install the OS through the OS already on it and over-ride the entire drive. (Yeah looking back at this I don't think this would work because externals don't have usb drives >_> but i'm sure someone can explain that part better and get the jist)