On my external hard drive, from Disk Utility, the 1st partition is FAT32, 2nd is Mac OS Extended, 3rd is ExFAT. I want to use the FAT32 partition to boot on another machine to install Windows on this machine but it doesn't boot. The thing is if I turn that Windows machine's hard drive to external hard drive and create a bootable partition then it is able to boot (this hard drive has two partitions all FAT32). So I guess the problem is my external hard drive with different types of partition. Does anyone know how to make my external hard drive to boot? I have searched a few but my problem is I have only my Mac, no other machines nor more hard drives.

Edit: I want to keep the external hard drive just for booting, and install Windows 8 on the Windows machine's hard drive.

  • 1
    Just so we're clear, you want to continue using the external drive as an external, and install Windows on it? Also, which version of Windows are you trying to install on a FAT32 partition?
    – Karan
    May 17, 2015 at 21:43

1 Answer 1


I backed up all data in the external drive, formatted the other two partitions to FAT32. It still didn't boot. I deleted all three partitions, created one new partition (doing these by clicking -/+ in Disk Utility) so the drive had only one partition FAT32. It still didn't boot. I found out I had to format the whole drive (the parent drive in Disk Utility, not the mounted partitions), now it is able to boot. So I think the reason is because my external hard drive was originally formatted as Mac OS Extended on my Mac.

It would be interesting if somebody found another way without formatting the whole drive!

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