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I know the BIOS emulation of the EFI firmware in Macs does not support booting from FireWire or USB drives, and the EFI version of Windows doesn't seem to be compatible with Apple's EFI implementation, so you can't install or boot Windows from one of those. But Thunderbolt drives are really just PCIe SATA controllers with one or more SATA disks attached, much like internal drives.

I have read that you can boot OSX from at least some Thunderbolt hard disks and SSDs. So is it possible to boot Windows off such a drive? And can you suggest any specific models of disks or Thunderbolt-SATA adapters that support this?

I realise that the Boot Camp assistant might not cooperate as the drive is "external", but the assistant is just for partitioning. You can prepare the disk's partition table manually, so the main question is if the Mac's firmware will boot an MBR-based OS from a Thunderbolt drive.

Obviously I could just go and buy a Thunderbolt drive and try it out but they're pretty expensive compared to otherwise equivalent and more universally compatible USB3 drives. Also, support is likely to vary between models, so chances are I'd pick up the wrong one.

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3 Answers 3

up vote 8 down vote accepted
+100

I’ve not tried MBR1… but: you can indeed boot Windows 7 and Windows 8 (64-bit versions) in EFI mode off of a Thunderbolt-connected disk. I have a handful of the Buffalo Ministation Thunderbolt2 drives with the original, slow 5400 rpm hard drives replaced with various SSDs, and they work wonderfully3. You can boot off of them by pressing Option during the chime at bootup, and they show as an orange-colored "EFI Disk" in the selection options.

Now, the trick is getting Windows installed on the disk in EFI mode, because when you stated that "the EFI version of Windows doesn't seem to be compatible with Apple's EFI implementation," I assume that means (like me), you tried, and it just refused to install. Well, to my knowledge, that's correct — however, the only part that isn't 'compatible' is the install process — whatever Windows tries to do to the EFI partition just before it goes to reboot doesn't work out right.

 

Solution to the rescue: install VMware Fusion on your Mac (even the trial will do) and use Vijay Pandurangan’s blog post to help you mount your external Thunderbolt drive directly to a new VM. Pay special attention to the comment at the bottom of the post: Hajo makes it much, much easier.

Partition the drive as GPT with OS X, and/or install an extra copy of OS X on the Thunderbolt disk first (if you wish) and leave free space for Windows. If you’re not planning to have a spare copy of OS X to boot from this drive, leave all the space blank (don’t partition beyond the EFI partition OS X will initialize with GPT).

Set the VM to boot with EFI, and install Windows 7 or Windows 8 (has to be a 64-bit flavor to support EFI) directly to the disk. Now, once Windows has gone through its setup process, and it counts down for a restart, shut the VM down. That’s right - you're done with Fusion4,5, and you can go straight to booting from your new drive. Seriously. Press Option on boot, and you will indeed see "EFI Boot" as an option, you can choose it, and Windows on Thunderbolt you will have.

I don't expect you would have any issue booting from any other standard Thunderbolt drive either, especially if it is an AHCI SATA drive like the Buffalo.

 

While I’m fairly confident that you could get plain-old MBR-based Boot Camp to work, why would you? EFI is the future, and once you go through the process, you can boot your Thunderbolt Windows disk from just about any new Mac5.

1 I haven't had the need for it, yet. There isn't an OS, or utility I've needed to run on my Macs that doesn't have EFI support, and I have an aversion to looking back and strongly feel MBR is looking back — like 1983 called, and wants its 10MB MFM hard drive back, back.

2 The original disk worked fine too, but gaaah! so sloooww.

3 Brian Klug wrote a really excellent review on this particular drive on AnandTech, and that's what convinced me to buy it. It's a beautiful piece of equipment, with fit-and-finish like Apple, comes with both a Thunderbolt and USB 3 connector and cables! to match. Yes, the included drive is slow as all get out, but you can replace it with just about any 2.5" HDD or SSD of your choosing. Just make sure you have a hair dryer.

4 Unless, you're not. I use both interchangeably. You can boot directly into Windows 8 on hardware, and you can boot into it from Fusion using this configuration when the need strikes you, like you're working on something, don’t want to reboot, but need to open a .PST file in Outlook 2013… for instance.

5 Windows licensing issues notwithstanding; you’re going to have to buy a copy of Windows for every machine you want to boot it on for long-term use in order to not get black backgrounds and "this copy is not genuine" type errors.

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Many thanks for the detailed answer! I wonder if this EFI trick would actually work for a FireWire or USB drive - I think I'm going to try that before buying a Thunderbolt drive! –  pmdj Dec 23 '12 at 13:19
    
An update: I tried the above instructions on my 2010 MacBook Air, using a USB HDD I'd partitioned as GPT with Disk Utility (FAT32/HFS+ split). It doesn't show up in the "alt" boot menu, but it does in System Preferences -> Startup Disk. If you select it there immediately after the installer reboots for the first time, it bluescreens with the inaccessible boot device code - the USB storage driver isn't loaded on boot. So I booted up in VMWare, let the setup run through to the desktop and ran regedit… –  pmdj Dec 27 '12 at 11:57
    
…In regedit, I changed the BootDriverFlags value to 4 in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control. While in there I also changed PollBootPartitionTimeout in ``HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PnP` to 30000 (decimal) in case it took a while to show up. Then shut down the VM, again selected Windows in System Preferences and lo and behold I'm booting Windows off USB on a Mac. However, try the same thing on a 2011 iMac, the machine I actually want to use it on, and it fails with no bootable device insert boot disk and press any key (grey on black screen). –  pmdj Dec 27 '12 at 12:06
    
I found a comment on the web somewhere claiming that the MacBook Air, as it lacks an optical drive, has extra firmware support for booting non-OSX systems from USB. The iMac in question has a DVD-RW drive, so I guess that would explain why I absolutely cannot get the damn thing to boot. I guess Thunderbolt might still work. –  pmdj Dec 27 '12 at 14:34
    
Okay, it seems that I messed up when setting the VMWare virtual machine to use EFI: I added the line firmware = "EFI" to the .vmx file. It turns out this is case sensitive and must be firmware = "efi" (lowercase). So I'm now retrying with that. It seems that the MacBook Air supports booting MBR-based USB drives! –  pmdj Dec 27 '12 at 15:20

The only way I found to make this happen was to just create a small (20 gig, smallest possible) bootcamp partition on the internal drive, then let Windows 7 setup start, and just select the external Thunderbolt drive (in my case, a Lacie Rugged with a Samsung SSD inside) as installation target.

Granted, it's not "pure external" as solution, but W7 definitly boots of the thunderbolt SSD that way, which is what you're asking.

Downsides : you can't reclaim that 20 gig on the internal drive : destroying it will render your W7 on the Thunderbolt useless. And your hibernation goes out of the window.

Upsides : huge speed improvement on W7. After drive recognition (which takes a while, but does so too on most PC hardware) W7 starts up in 15 seconds flat. And as you're installing W7 "the right way", you don't have to worry about ssd alignement or optimalisations : W7 does most (not all) of those by itself, including support for trim, disabling defrag and that kind of stuff. The solution turned out tot be stable even after a couple of months working.

I've tried all other methods that involed cloning bootcamp in some way, but results were negative on a Imac 27 late 12.

I was a bit worried about power ratings of the buspowered Lacie Thunderbolt, but the used Samsung SSD has a measured max power drain of about 3.5 Watt in fully stressed write and 0,35 or something in idle,. The Thunderbolt port is rated to 10Watt, so that shouldn't be a problem, not even in the long run.

It looks a bit odd having that Lacie around as W7 boot disc, but it works very well and does the Imac justice running Windows, as performance on the internal fusion drive was abysmal in regards to the Imac's harware.

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Other people have got it working:
http://forums.macrumors.com/showthread.php?t=1351547

Or if you have a retina MacBook Pro:
http://forums.macrumors.com/showthread.php?t=1414769&page=5

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