That external hard-drive...

You may already have discovered this, but I'll mention it anyway - an external hard-drive can double the capacity of your existing hard-drive, but you can't use it to install more operating systems. That's because it's connected by USB and that's too slow for normal operations in an operating system. And Windows will tell you that you can't install one of their operating systems on a removable device. That would make it entirely too portable and probably too handy, and they wouldn't be able to keep track of your MAC Address, because that would change whenever you plugged it into another computer - and that's probably the real reason it's a 'no-no'. Licensing problems. 

They could solve that by giving each portable drive a special boot sector which would include its own MAC Address, but nobody's thought of that yet....or if they have, they aren't doing it. Perhaps because it would be too much like the Remote Access already in use. In other words, complicated and a security risk.

Too bad - I could have three more operating systems on here, if it worked... and computers still have further improvements to be reached. I tried this, because I'd like to be able to install the test beta of Win-10 on its own separate drive, so that it wouldn't have to interact with the rest of my systems in order to do its own thing. I'm not too happy about the way it grabs control of the main Boot Sector and subordinates everything else under it. It's just a test program, after all, so why should I trust it to manage other operating systems' booting control? For me, that just doesn't make sense. That's like letting an inexperienced kid drive the bus for you.