External hard drive

i have no i idea about that but i think yes. can you install another operation system on your external hard drive, because it is also a hard drive but the only difference is the external hard drive is removable.

Yes you can, but if you wanted to use that operating system as your primary or default operating system you need to set up that external drive as your primary drive. Once it is the primary drive, then your PC will start looking for operating system in that drive and since you have Linux installed in there, then it will boot on Linux.

The drive letters are assigned automatically by windows by the next available. If you plug your equipment into your PC in the SAME ORDER every time you start your PC ( or even leave them attached), then they will be assigned the same letters.

yes you can, as long as your external hard disk is bootable. but if you just want to install linux to your PC, i would suggest that you don't need an external hard disk. linux and windows can "live" in the same hard disk :).you just need to install it in a new partition. better make a backup of your files before installing.