Is El Capitan all about interoperability with other equipment ? I don't use any of that nonsense, I can't see why I would want to sync my phone with my mac, nor would I wish to download from my mac, to a phone, from a distance. In anycase the mac at home would probably be switched off anyway. As for Cloud, expensive nonsense, Time Machine costs nothing, but just for extra security I keep a smaller external HD, to double record the bits I would not wish to lose, if both the mac and Time Machine crashed at the same timeFinally!
I installed windows with no problems a few months ago. Last night I upgraded to El Capitan. Mac worked ok in El Capitan. The problem was when I used boot disk to go to windows. I couldn't log into widows because wireless keyboard wasn't recognize. Mac support had me get a wired keyboard with a usb connection. I was then able to log into windows. After trying several things to get the Apple wireless board to work in windows, they finally had me change a driver in windows device manager. Now the wired doesn't work. No way to get past the windows login screen. Will be calling Apple support again tonight.
I've been running Windows on an external Thunderbolt drive for ages. You can't do it with the Bootcamp installer, but you can do it on your own. Macs have no problems booting from external Thunderbolt drives (USB has had issues in the past, not sure if it's still the case).
I do the opposite. I remove clunky OSX and install Faster and more productive Windows on it. Someow Windows runs better on an iMac!
Unfortunately, no. Every major OS release I test it again. And the odd thing is, it's not every receiver and projector that causes this issue. So. Annoying. ;-)
