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In the past I’ve been able to run Windows 8.1 on a Mac Book Pro Retina Mid 2012 installed on an external hard drive using WinToUsb. I had OSX Mavericks installed on the internal drive. After turning on the machine I could simply press „Alt“ to see all bootable drives currently connected. I could then select the Windows drive to boot into it.
I could even boot into it on another Mac Book Pro Retina Mid 2012 running first Mountain Lion and then Yosemite using the same procedure.

I now bought a new Mac Book Pro Retina 15“ Mid 2015 and tried to boot into Windows, but the drive doesn’t show up next to the OSX drive. I’m now wondering what causes this and only know the hardware has changed and it came with Yosemite preinstalled.

How can I fix this and is it still possible to use external Windows drives with this new Mac at all? What exactly is causing this, the new hardware or operating system?

Apple support couldn’t help and say they only support Boot Camp installations on internal drives. Installing to the internal drive is no option for me, since I would like to keep the old Windows installation without reinstalling and I like to save the expensive internal SSD space.

Thanks a lot in advance!

Update

I had the chance to test on another Mac Book Pro Retina Mid 2012 which was just updated to Yosemite. I could successfully boot into the external Windows installation on this machine. I therefore guess it's a change in hardware and not Yosemite causing the issue.

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