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I'm wanting to install windows on my mac. I have a 1tb hard drive and I won't need that much space for it, and I want to use the drive for other things. Is it possible to have one part of the drive allocated to windows and the other to backups? I have already tried creating 2 partitions in disk utility (one mac os x extended journaled) and the other was ms-dos (fat). How can I do this?

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  • Microsoft's Windows no longer uses FAT, for it's own file system, it uses NTFS. Jul 24, 2015 at 14:28
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    Consider also Virtual box, it will allow you to run both at the same time. You could also run a Gnu/Linux as well. Jul 24, 2015 at 14:29

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"Boot Camp" is a bit ambiguous, since it's really several tools and techniques wrapped in (or set up by) an Apple utility. What's more, some people refer to their installations as using "Boot Camp" even when they ditch most of the critical features of Apple's utility; in this context, the term is being used (confusingly for everybody involved) in a way that's more-or-less synonymous with the more generic "multi-booting." I'm pointing this out because there are a lot of variables to consider, and you might want to deviate from a true Boot Camp setup.

Windows Installation Mode

The first, and most important, variable is the Windows installation mode, which can be either of two things:

  • BIOS/CSM/legacy mode -- This boot mode is what Boot Camp helps you set up. In this mode, your disk uses a hybrid MBR (more on that monstrosity shortly) and Windows uses a BIOS-mode boot loader. The hybrid MBR both tells the Mac firmware to prepare its Compatibility Support Module (CSM; the part of the firmware that emulates a BIOS) and makes Windows treat the disk as an MBR disk, despite the fact that OS X treats it as a GPT disk. This mode is required for Windows Vista and earlier, and the vast majority of Windows 7 installations use this mode, too.
  • EFI mode -- Users have reported that Windows 8 and later can install on many (perhaps most) Intel-based Mac models using EFI mode. Some people have gotten Windows 7 to work in this way, too, but doing so is a major undertaking. When set up in this way, your disk must not have a hybrid MBR. This is cleaner and safer, but ironically, it can be a complication, since OS X's Disk Utility and Boot Camp will both create a hybrid MBR if you use them to prepare FAT partitions.

You didn't mention what version of Windows you're trying to set up or what Mac model you've got, so it's impossible to tell you which mode you should use. As a general rule, though, if you're using Windows 8 or later, my inclination would be to at least try to do an EFI-mode installation. This will require ditching most of Boot Camp (you may still be able to use some Windows drivers), but it will be much cleaner. Most importantly for you, using EFI mode won't impose the limitations and dangers of hybrid MBRs.

OTOH, if you want to install Windows 7 or earlier, or if you have problems with an EFI-mode install, or if you don't want to take the time to try an EFI-mode installation with no guarantee of success, using Boot Camp may be in order. You'll need to understand hybrid MBRs (or be at the mercy of chance) in this case.

Controlling the Windows boot mode when installing the OS can be tricky. The installation discs for Windows 7 and later ship with both BIOS and EFI boot loaders. Which one is used depends on the firmware or boot manager you use to boot the medium. You've got to be very careful when you boot your Windows installer, since Windows will try to install the boot loader for the mode in which it was booted. If you boot the installer in EFI mode, Windows will refuse to install to a disk if it uses an MBR (or a hybrid MBR); and if you boot the installer in BIOS mode, it will refuse to use the disk if it's GPT without a hybrid MBR. Unfortunately, I don't know offhand how to force a particular boot mode using Apple's boot manager.

Hybrid MBRs

On a single-boot Mac, the disk uses the GUID Partition Table (GPT), which is a (relatively) new partitioning system that's far superior to the older MBR system. Part of GPT is a "protective MBR," which is basically a fake MBR that makes the disk look as if it's fully-occupied by a single partition to MBR-only tools, thus providing some degree of protection from damage. In a hybrid MBR, this protective partition is shrunk and up to three GPT partitions are mirrored in MBR data structures. This enables Windows to see the disk as an MBR disk even while OS X uses it as a GPT disk.

The main danger with hybrid MBRs is that the GPT and MBR data structures can fall out of sync. This is most dangerous when somebody uses a GPT-unaware tool to resize or move an MBR partition. I can't tell you how many times I've walked people through recovering from such a blunder.

Of most relevance to your question, though, Apple's tools generally create a hybrid MBR by skipping the EFI System Partition (ESP) and then adding the three following partitions to the MBR. In the past, this would have been a single OS X partition and then up to two Windows partitions; however, changes in the last two or three versions of OS X mean that it's common to see two OS X partitions in the hybrid MBR, leaving just one for Windows.

You aren't limited to Apple's tools, though. My own GPT fdisk (gdisk) can create hybrid MBRs in whatever way you want; you could put up to three Windows partitions in the hybrid MBR, for instance. My rEFInd boot manager comes with a utility called gptsync that tries to set up hybrid MBRs intelligently and automatically; but its algorithm might or might not be optimal for you. (Note that rEFInd's gptsync is a modified version of rEFIt's gptsync, which used an algorithm that was similar or identical to Apple's. Also, in order to avoid accidents, gptsync isn't accessible in rEFInd unless you edit the refind.conf file and modify the showtools option.) I wouldn't be surprised if other third-party tools support hybrid MBRs, but I don't know of any off the top of my head.

Overall, if you go with a hybrid-MBR/Boot-Camp setup, I recommend you read my reference page on hybrid MBRs to learn more about them. You should then be in a better position to design your layout and set it up.

Boot Managers

Your question doesn't explicitly mention boot managers, but any dual-boot configuration requires one. A simple boot manager is built into the Mac firmware; you hold down the Option key to select your OS. This is adequate for many people. If you want something more, you can install something else. In the past, this was often rEFIt, but it was abandoned five years ago, which is why I forked it as rEFInd. Both rEFIt and rEFInd support dual-boots of OS X and Windows, using either BIOS/CSM/legacy-mode and EFI-mode Windows installations. If you're triple-booting with Linux, you might also consider GRUB 2, which most Linux distributions use by default.

Alternatives

In a comment, Richard suggested using VirtualBox rather than setting up a dual boot, and this is indeed worth considering. Virtualization software (VirtualBox, VMWare, etc.) works quite well these days. It has the advantage of being safer and easier to set up -- there's no need for a hybrid MBR or third-party boot managers. You can also run both OSes simultaneously, which is often very helpful. Virtualization has the drawback of giving indirect access to your hardware in the virtualized OS -- your video performance will suffer, and you'll have less RAM available. These problems can make virtualization a non-starter if you want to run Windows for gaming, but for many other purposes it works at least as well as a true dual boot.

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