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I have a mid-2011 Core i7 Macbook Pro that currently is dual-booting a File-Vault encrypted OS X Lion installation and an OS X Snow Leopard installation. Unfortunately I need to keep both of these on the internal drive which I believe prevents me making yet another partition for Windows 7 via Bootcamp because of some limitation on the number of partitions (though I could be wrong on this).

I only need to run Windows 7 natively (for gaming, etc) every once and a while and what I would ideally like to do is install Windows 7 onto an external USB or Firewire drive and boot/run Windows from the external drive when needed.

I have successfully installed Windows 7 by temporarily swapping my internal drive and for my "external" drive. After installation Windows boots fine while the drive is inside the laptop, but when I move it to an external USB enclosure it is not listed as a boot option either from the startup menu (holding down Option on boot) or from the 'Start-up Disk' System Preferences.

Is it possible to run Windows 7 from an external drive? If so how?

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Late 2011 MBP? We've barely entered the second half of 2011, and MBPs haven't changed since, like, April. Do you mean early 2011 or mid 2010? (There were no late 2010 MBP changes.) – Spiff Sep 8 '11 at 2:07
It was purchased in July 2011, I've updated the question to reflect that. Thanks. – Adam Franco Sep 8 '11 at 12:31
I think taking your main HD out and installing Win 7 to a new HD would be fine, although the problem would arise when starting up. You'd probably have to hold down the key to choose which device you want to boot from, or the bootloader might have a fit and refuse to work. Also, USB would probably be too slow for a (gaming) OS, but firewire should be OK. Could you not just try it? – tombull89 Sep 8 '11 at 12:56
Thanks, I've now successfully installed Windows 7 by swapping the internal drive (and updated the question), but I still can't choose it as a boot option when it is in a USB enclosure. – Adam Franco Sep 9 '11 at 1:27
up vote 1 down vote accepted

Windows 7 does not support installation onto an external HDD and is incapable of running of a USB HDD on Mac or PC hardware. As I recall, this has to do with how Windows handles USB on boot.

Microsoft Technet Forums

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You are correct, and it applies to bootcamp as well: You can use Boot Camp to install Windows on any internal hard drive, but not on an external hard drive. – G Koe Mar 10 '12 at 7:24

You cannot do a bootcamp boot from an external drive @dustin-G is correct.

You can triple boot from your internal with rEFIt, but you have to manually partition the drive using diskutil in order to get the necessary partition structure for triple booting.

As if that wasn't bad enough, it gets worse. MBR only supports 4 primary partitions and GPT does not support extended partitions.

Combining these two limitations means that a dual partition system disk can have a maximum of 4 primary partitions. In addition, Apple reserves the first primary partition for the "EFI System Partition". That leaves us with only three partitions to play with, one for each OS. Also, for some reason Boot Camp expects the Windows "C:" drive to be the last partition present on the drive. If it isn't, Windows setup will crash with a "cannot find hall.dll" error after the first reboot.

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