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I have an external drive that uses the GUID partition scheme, and has the partition info mirrored to an MBR as Macs are won't to do. The drive does FireWire and USB, and Ideally, I'd like any solution presented here to work with both buses.

I have HFS+ and NTFS partitions on it. The NTFS are among the first four as MBR needs them to.

I did this partitioning via Disk Utility, and then used Winclone to copy my Boot Camp partition to the NTFS partition on the external drive. The cloning went fine, the files are there, but the Windows partition does not appear to be bootable.

I tried using rEFIt, by keeping it on another partition on the external drive and booting from it, but that was to no avail. I still got a "No bootable device…" message.

The only way that I was able to boot from the external drive was by keeping a minimal Windows installation on a partition in the internal drive. If you scour the web you'll find a way to do this using only a minimal partition (60mb or so) with only the very necessary files there. But I really wanted the external drive to be reusable among many Macs.

update: it has since come to my attention that Windows XP does not boot out of the box from external devices, but that it can be hacked to do so. My googling and reading shows people doing new installs to achieve so. And it's all very clumsy and there doesn't seem to be a definitive way to do this[1]. Can I hack an existing Windows XP install to be USB and FireWire-bootable? Also, most solutions focus on USB thumb flash drives, and I'm talking about real spinning harddisks.

So, most importantly, is it at all possible to boot Windows XP on a Mac from external drives?

I have spare drives aplenty to play with, and so I have been doing, but so far I've mostly managed to mess up my internal disk partitioning scheme in a very interesting, peculiar and irrelevant way. (SuperDuper to rescue). So far, all FAIL.

[1]: The Mac of Windows hacking, so to speak.

share|improve this question
From what I've seen so far at least a friend's Macbook wouldn't boot from USB. – Joey Jul 31 '09 at 10:44
Macs can certainly boot from USB into OS X, I don't know about Windows. – MJeffryes Jul 31 '09 at 10:56
I can boot OS X from USB. But that's not the issue. I use FireWire most of the time anyway. – kch Jul 31 '09 at 12:25
For anyone interested, check this out. There must be some files modified in the Windows installation, or it wouldn't boot from USB once installed. – user46750 Aug 19 '10 at 9:43

If you install rEFIt it will give you a boot option menu on every boot in which you can select any partition or external device to boot from.

share|improve this answer
+1 rEFit is the must have tool if you dual boot on a Mac – BinaryMisfit Jul 31 '09 at 11:14
This sounds good, but I'm wondering if there's a way to do with with an off-the-shelf mac, since the whole point of having an external drive is to use it with different machines. In that regard, my question is more in the lines of "am I doing it wrong?" – kch Jul 31 '09 at 12:27
If you can see the drive as a Startup disk in MacOSX preferences then you can do it from there. You can also trying holding Alt or Cmd when booting but neither are good solutions and often fail. – BinaryMisfit Jul 31 '09 at 13:30
Hi, I installed rEFIt on a partition on the external drive, and wincloned my internal drive Windows installation to another partition on the external. I'm able to load rEFIt and it shows the Windows partition, but once I choose it, I get a black screen with an error message which I now forget, something about unable to load OS. – kch Aug 3 '09 at 9:18
message would be: "error loading operating system" – kch Aug 3 '09 at 9:28

I wanted the multi-tool of USB drives. Two partitions: one bootable Mac partition and one that could be read from and written to by both Macs and Windows machines. I finally got it done and I don't know why it works, but it does. Here's the deal:

Equipment needed:

  • A 16GB USB drive or larger (try to find one that is fast too)
  • A drive or partition with a minimal OS X build of your liking (10.6.7 for me was 7.2GB)
  • A running Mac
  • A Windows (virtual) machine - I used Parallels 5 with Windows 7

The procedure:

  1. Partition the USB drive with GUID partition table; the first partition should be FAT32 (or exFAT), the second should be HFS+ (8GB minimum to fit OS X)

  2. Plug the drive into the Windows machine and agree to it formatting the drive (200MB only for some reason) and label it something odd (I use xxx)

  3. Plug the drive into a Mac, fire up Disk Utility, click on the USB drive, select partition tab, select the xxx partition and press the "delete" key. You are now back to having two partitions.

  4. Restore (using Disk Utility) your prebuilt minimal OS X to the HFS+ partition.

  5. I like to do a Disk Utility repair on the HFS+ partition following the restore.

You now have a Mac-bootable USB drive that has a Windows partition which is readable and writeable by both OS X and Windows.

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From my earlier comment.

When using Bootcamp the correct method of booting into Windows is to go into System Preferences - Startup Disk and select the partition to reboot from. If your FireWire is seen as a listed drive then you can set it here and reboot.

However this seems to break intermittently and therefore rEFit is the best solution I have found to do dual booting with on the Mac.

share|improve this answer
Actually the holding option during start up generally lists exactly the same drives that the Startup Disk prefpane does. Unfortunately the partition on my external drive is not listed. I'll first try another external drive with less complicated partitioning scheme and see if I can make it work. – kch Jul 31 '09 at 22:59

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