Running El Capitan on an external drive?

Thanks, do you mean the old iTunes that can't be updated on my Snow Leopard MBP will be affected if I use the iTunes on the external drive? That's partly what I need it for as the old iTunes won't back up iPhone anymore on older iTunes as you know, it won't let me update iTunes on that OS. When booting from external I thought it was a separate programme and wouldn't interfere with the other one which isn't actually open? I'm not getting this like I thought I may do. I obviously am messing with stuff I don't understand by the sounds of it.

The El Capitan wouldn't have any of the stuff that's on Snow Leopard on the laptop would it? It would be as new start up?

Won't it be on screen when SL is not booted, I would have choice of SL or EC?.

It would be a separate 'Mac' so to speak or I'm reading this wrongly?

Thank you, Yes, that's what I understood, so if they're both separate entities, in what situation would this occur, this is what I don't get?

'....Be careful which files you open when running ElCapitan. For example, if you're running an older iTunes version and you open you library with a newer version on El Capitan it will irreversibly update your library file so it can no longer be opened with the older version.'

Thanks for your reply but I don't actually understand any of that. I understood that the only reason to press option key was to choose which OS on start up, and then I'm in that one, so there is iTunes in the El Capitan on the external, and iTunes on the Snow Leopard on the machine if I open that OS? And they're separate entities? That's what I thought anyway, but now I'm just totally confused. Why would I have to launch iTunes with the option key if I'm in or other of the OS, internal or external, if you know what I mean? Doesn't it just launch when I open it according to whatever OS I'm in. I can't be in both at the same time, if I boot each one separately?

Thanks for your reply but I have no clue what you're talking about.

I wanted two operating systems, and two separate Macs so to speak running different OS and thought that was it, and I'm now beyond confused as to anything you've said, I have no clue what you mean. They're either different separate OS running their own separate iTunes, or they're the same. I give up. I don't think I've ever been this confused about something I thought would be relatively simple. Don't think I'll bother now

I wouldn't know how to tell it to, And why would I? It makes no sense to me, sadly.

I just thought one OS was on the external, and one on laptop. And they were both unrelated. But now this has just confused me no end. Thanks anyway, but I'm obviously not up to speed on doing any of this stuff.

I actually wanted the external El Capitan for the later version of iTunes. So I can't start a new separate from the internal iTunes, new version of iTunes on the external. Just don't get why it would conflict with each other. That's what I don't understand. If you can't have a separate version on the separate external OS, then it's not going to help me. I get what you're both saying, it will interfere with the old internal iTunes, but I can't get why, if the external is supposedly a new and separate from the internal. Do you get what I'm not grasping here? Why is is sharing anything with the old OS?