I spend about 1/2 the year in the US and the other half in the EU. Instead of bringing a computer with me when I travel, my plan is to have a computer in each place and just carry an external drive and backed up Time Machine files with me as I go from one place to the other. For example, I would back up my computer in the EU using Time Machine, travel to the US and update the 2nd machine with the Time Machine backup from the 1st machine. When I go back to the EU, update the EU computer with the files I was using in the US, rinse and repeat. Here is the dilemma. I bought a new Mac Mini as my 2nd machine, and am using a early 2009 MacBook Pro as my 1st machine. My Mac Mini is running Sierra however the newest OS I can run on my 2009 MBP is El Capitan. My assumption is that I would have to be running the same OS on each computer, but again, that's just my assumption. Since I can't run Sierra on my MBP, I was thinking of loading El Capitan on the Mac Mini so the OS on both computers would be the same. The idea is clear to me, I hope I was able to explain it adequately so it's clear to you. Is what I'm proposing seem realistic? Is there a better way to do this? I'm open to suggestions. thanks in advance.