I have a Mac Os X Capitan 10.11.3 Mac Pro Late 2013. I installed Windows to my Seagate 1TB USB Hard Drive but my Mac can't boot it. It does show up on Startup Disk on system preferences. I installed Windows 7 to my USB using Parallels and by following this guide https://www.youtube.com/watch?v=4h9NSq1zYFI I already have Windows 10 partitioned on my Mac using bootcamp but because my Mac only has 251 GB of storage space and 32.8 GB of it is being used to run bootcamp Windows 10. When I boot into Windows 10 I hardly have any space at all. To fix that problem like I said I installed Windows 7 to my Seagate 1TB USB Hard Drive. To fix the problem of not being able to boot Windows 7 from my USB Hard Drive I was thinking of removing my Windows 10 partition from my Mac. Would it fix the problem? Cause right now I am thinking that bootcamp is blocking me from running Windows 7 from my USB Hard Drive. Or is it that my Mac is not compatible with running Windows 7 from my USB Hard Drive and that I should install Windows 10 on my USB instead? I did try to install Windows 10 to my USB using WinToUSB but I got the same result it was unable to boot it. If someone could please explain why my Mac can't boot Windows from my Seagate 1TB USB Hard Drive that would be great! Thanks a lot!