Running El Capitan on an external drive?
Is is possible to install and run El Capitan on an external drive, while running and keeping my old Snow Leopard 10.6.8, and still run Snow Leopard on my 7 year old MacBook Pro, or doesn't it work like that and both have to have same OS running? I just want to keep the existing OS on the laptop that I know and use, where so much isn't compatible with later OS, but want to check it out anyway, hence can it be done and can I use El Capitan alongside it on the external? Thanks for any help
MacBook Pro