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I’ve got an external USB drive that’s a bootable clone of a Windows XP hard drive (theoretically — I haven’t confirmed that a PC can boot from it).

I’m trying to get my Intel Mac mini to boot from it. I’ve tried plugging in the drive, then holding down alt as I start the Mac, but it doesn’t show up in the list of bootable drives. (I’ve got a working Windows XP Boot Camp partition on the Mac mini’s internal drive, as well as the Mac partition).

This KB article suggests it may not be possible to boot the machine from it.

Anyone know a way to boot from the external USB drive?

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Before you go through a lot of trouble, make sure that the USB disk is actually bootable. Preferably test it on a (non-Mac?) PC that you know can boot from USB. Then you've eliminated any problems with the USB drive. –  Torben Gundtofte-Bruun Aug 18 '10 at 13:18
    
Sure, that’s a good idea. Hopefully I’ll have access to one next week. –  Paul D. Waite Aug 18 '10 at 13:59
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Depends on which model Mac Mini you have. Earlier versions of Intel Macs don't support booting from USB drives as far as I know.

The only other possibility would be your partition structure. Intel Macs use the EFI boot loader, and these prefer the GPT partition structure rather than MBR. If you look closely enough at a drive formatted by Boot Camp Assistant, you'll see a very small FAT32 partition at the beginning of the drive. This is so the partition can be "blessed" and marked as the preferred bootable volume on the machine, so if you restart it automatically boots to that OS. If you cloned the disk from one that you used in a PC, it's most likely formatted MBR.

However, I have verified that it is possible to boot to MBR Windows volumes by holding down the Option key. Only thing is the Windows volumes don't show up until later. However, I'm not sure if this works at all via USB. I would think not.

First step in troubleshooting would be to check and see if your Mac Mini can boot to a USB volume at all... i.e. use Carbon Copy Cloner or SuperDuper to clone your Mac volume to a USB drive and see if you can boot from it.

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Sure. I’ve got the latest Mac mini, bought it a month ago. Good suggestions, and interesting info about the partition structures — my cloned USB PC drive probably is MBR I guess. I’ll see if I can find another drive and do some experimentation. –  Paul D. Waite Aug 18 '10 at 13:59
    
Yeah your Mini supports booting from USB. I think cloning Windows without copying the partition structure is going to be tricky. One thing you can do is use Winclone and create a disk image of the MBR Windows disk, then use Boot Camp Assistant to erase/prepare the disk for Boot Camp (not sure if it'll use USB disks, if not use Disk Utility), then clone the disk image back to the new volume. –  churnd Aug 18 '10 at 21:12
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There is one thing that I've noticed myself when trying to boot from an external USB drive, is that if the drive itself has an IDE interface, then the mac mini will not recognize it. But if the drive itself has an SATA interface, it will.

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