That would work if I only used my computers inside of my house, which would defeat the purpose of having an external drive.
I'll be going to a college in a different city and I'll have one computer at home, one computer in my rented apartment and a laptop to carry around.
I could set up a file server at home, but why would I want to be downloading files if I can have immediate access to them?
Besides, I won't have unlimited bandwidth in my rented apartment.

I just need the best way to have rw access to my external drive across those three operating systems.