There is a comprehensive guide for Thunderbolt drives and at least one person reporting that they successfully installed windows 8 onto a usb drive and run on a mac, however I can find very little info on this - have any of you achieved this? 
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Have you successfully installed window 8 onto an external USB 3.0 drive?
- Thread starter tmanto02
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It's firmware specific. At least 2008 and older Macs have a CSM-BIOS that doesn't support USB booting. Newer Macs have a CSM-BIOS that apparently does, or it wouldn't be possible to boot from a USB stick as an installation source. But I don't know how good Windows is at installing onto a USB destination. Making Windows transportable is not a design goal of Windows, in fact it really dislikes being moved around especially to different CPU families than what was used for the original installation.
Yeah the problem so far has been on the windows end. The traditional install method does not support usb drives. There is a thunderbolt work-around but not a USB one from what I can find.
It requires explicit firmware support, as well as explicit OS support. I can USB boot linux on my 2008 Macbook Pro in EFI mode, but it fails in CSM-BIOS mode, the USB bus simply vanishes (or is reset right in the middle of early boot, it's unclear which).
I have a 2012 cmbp
Can I install either windows 7 or 8 from an external usb 2 or 3.0 drive?
Thanks
Can I install either windows 7 or 8 from an external usb 2 or 3.0 drive?
Thanks