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After using a dual-boot scheme successfully for a while, I'm now trying to add Windows 10 (Pro) as a third OS to my Mac mini (rEFIt).

The order of the most(?) important steps so far:

  • 2006: buy Mac mini with OS X
  • 2012-ish: install rEFIt
  • ... and install Lubuntu (currently 15.04), brings GRUB2 with it
  • this week: add RAM (now 2 GB) and storage (replaced original HD by 120 GB SSD)
  • install Windows 10 (Pro) in unassigned free space

As described in this askubuntu question, I can't get Windows to install properly. It seems that I have too many primary partitions.

Given that the current state is like this screenshot of GParted, and Windows will certainly (again) complain that there are already four primary partitions when I'll just try again to install it in the unassigned free space:

What can I do (now) to finally have a "triple-boot" system?

Apparently it is not uncommon (or at least not impossible) to have such a system, but I'm currently having a hard time with the details. Do I have to change partitions (or details thereof), and if so, which tool on what OS is good enough for that job?

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It's critical that you understand a few preliminary points:

  • rEFIt is abandonware -- The last new version of rEFIt was released in 2010. Although it still works, you may want to consider switching to rEFInd, which is my fork of rEFIt that's still being maintained. rEFInd has some features that might be helpful to you, such as the ability to boot a Linux kernel directly.
  • Your Mac is probably 32-bit -- The first generation of Intel-based Macs used 32-bit CPUs, and the next generation used 32-bit EFIs but 64-bit CPUs. In both cases, the EFI boot loaders are 32-bit, which is unusual and limits your options in some cases. For instance, few Linux distributions officially support 32-bit EFI-mode booting, although it can be made to work in various ways.
  • Your disk almost certainly uses a hybrid MBR -- A hybrid MBR is an ugly and dangerous hack that Apple uses to support booting both the EFI-based OS X and BIOS-based versions of Windows. To triple-boot a system with a hybrid MBR, it's imperative that you thoroughly understand what's going on, lest you wipe out your disk and all your data! You can read more about hybrid MBRs on my page on the subject.

On most Macs, the preferred way to install Windows 10 is to do so in EFI mode. This would require replacing your hybrid MBR with a conventional GPT protective MBR. This in turn would eliminate any concerns about primary vs. logical partitions, since those are MBR concepts that don't apply to GPT disks. (Note that your GParted screen shot shows five partitions, none of which is extended or logical. This is because GParted favors the GPT data structures when it sees a hybrid MBR disk.)

That said, I don't know offhand if it's possible, or preferred, to install Windows in EFI mode on a 32-bit Mac. It could be that you'll need to stick with a BIOS/CSM/legacy-mode install of Windows on your system. If so, you'll need to preserve the hybrid MBR; however, you can modify it so that only Windows partitions are "hybridized"; there's no need for either OS X or Linux partitions to reside in the hybrid MBR. My page on hybrid MBRs, referenced above, describes how to do this; however, I strongly advise you to not muck with this until you fully understand it. The risk of badly damaging your disk should not be understated!

Whatever you do, keeping Linux booting may require additional changes. rEFInd can help with that, since it can boot Linux without involving the BIOS-mode version of GRUB that I suspect you're currently using. This will obviate the need to get GRUB and a BIOS-mode Windows boot loader to coexist. You might want to start out by downloading the USB flash drive version of rEFInd. You can then boot it and see if you can get rEFInd to boot Linux in EFI mode. (If the directory /sys/firmware/efi exists once you've booted, you've booted in EFI mode; if it's absent, you've probably booted in BIOS mode.) If this works, you can install rEFInd to your hard disk, and then either:

  • Use gdisk to create a fresh protective MBR -- type x to get to the experts' menu, then n to create the new protective MBR, and then w to save your changes and exit. Then try to install Windows in EFI mode; or....
  • Use GParted or OS X's Disk Utility to repartition the disk for use by Windows, then use gdisk to create a fresh hybrid MBR that holds the new Windows partitions, then install Windows in BIOS/CSM/legacy mode.

Absent other information, I'd try the EFI-mode Windows install first; however, it's worth trying to discover if this is the best approach for a 32-bit Mac. Note that the fact that you're (presumably) running a 32-bit computer is a critical detail; advice from a 64-bit perspective may not be applicable in your case.

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