Triple boot- on a mid 2010 iMac11,3 Having tried this before and rendered the mac partition unbootable, I decided to try again. 

With help, I managed to retrieve my data, wipe the HDD and reinstall Mac OS Sierra via internet recovery. Now I am trying to reinstall Windows 10 and then Ubuntu 16.04 but am stuck at the first hurdle.

I tried to use Boot Camp assistant to create a new partition for Windows but was informed that Windows 10 is unsupported on this Mac (Windows 10 was previously running on this machine but I had updated from Windows 8.1). No problem, thought I, I'll just dig out the Windows 8.1 DVD... Same message as before: "Windows 10 cannot be installed on this Mac" eh? Not wishing to give up, I made a FAT partition using Disk Utility. 

I then made another partition for the future Linux install. Feeling optimistic, I booted from the Windows 10 DVD in legacy mode and got to the point where you choose which  partition to install Windows on. I keep getting the message "Windows cannot be installed on this partition" even when I format or delete the partition to leave free space. 

Last time I "solved" this problem by booting in EFI mode and installing that way. This broke the partition map and left Mac OS unbootable. 

What is the best way to install Windows on the partition without destroying the ability to boot Mac OS too? Really, I would like to then install Ubuntu on the other partition but one thing at a time...

I have read other posts on this topic but the solutions suggested do not seem to work in this situation.

Output from Diskutil list:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Mac OS                  1.5 TB     disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:       Microsoft Basic Data BOOTCAMP                514.0 GB   disk0s4


The iMac has 8GB of RAM and I want the OSs to have roughly the space shown above. I have not started to install Linux yet and the Windows has failed to install so far.

OK so I have managed to install Windows 7 via bootcamp and then update to Windows 10, good so far, both OSs booting. Then I re-sized the Bootcamp partition from Windows to make space for Linux but guess what? The partition editor won't let me create a new partition in the unallocated space and Ubuntu won't install into it. I have already got 4 primary partitions and no more is possible. Still, I've got two OSs working again...I have updated the diskutil list and it doesn't show the resized windows partition. Is there any way of installing Linux in the free space without wrecking everything?

OK, so I went ahead and made 3 new partitions using Disk Utility and installed Ubuntu 16.04 onto them following instructions from 
Installing Ubuntu on Mac with macOS and Windows already installed
 Result: Mac and Ubuntu still boot fine but Windows now won't boot and gives a message " Missing Operating System" when I try to boot from it.

This is the new diskutil list output:

    /dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Mac OS                  1.3 TB     disk0s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk0s3
   4:                        EFI                         6.5 GB     disk0s4
   5:           Linux Filesystem                         198.7 GB   disk0s5
   6:                 Linux Swap                         15.2 GB    disk0s6
   7:       Microsoft Basic Data BOOTCAMP                514.0 GB   disk0s7


Anyone got any ideas for how I can rescue the Windows install?
Whatever happens, don't worry, because

You only lose what you haven't backed up™

So, if you don't have a backup of your system, make sure you create one:


Download and install "CloneZilla", a FOSS disk cloning utility, on a USB drive, in UEFI mode. Google is your friend, but this is a guide for you
Now attach a blank HDD the size of your hard drive and reboot your Mac in Clonezilla
Clone your internal hard drive over to the backup disk (make sure you don't do the other way around). The interface is pretty intuitive, but the procedure could take a while, regardless of how much data you have on your disk.


Now, make an ExFat partition on your disk using Disk Utility, launch the Windows installer, format it and install Windows.

Apparently in your case this seems not to be working and - in EFI mode - Windows messes up your EFI system.

I have a couple of suggestions.

The first: install Windows on a secondary HDD. This makes everything much easier, and you don't have to deal with countless other issues such as messed up partition tables and wrongly erased volumes. IIRC a secondary Hard drive can be installed in your system and - considering how cheap they are - it's really worth it.

The second, and most complicated (and maybe dangerous) is to run an Ubuntu (or any other distro) live session, open GParted, unmark your EFI partition as such (making it not appear as an ESP, but rather as a generic partition) and create two ExFat partitions. The first of them has to be very tiny (~200 MiB) marked as ESP, the other one - instead - should be as big as you want your Windows C: to be. After this is done, install Windows in EFI mode to the big ExFat partition (after having formatted it), then re-run GParted and re-mark the original ESP as such again. Now you can install rEFInd from macOS and boot your system in Windows by selecting the secondary ESP you created upon boot. If so you want, you can tweak rEFInd to boot an OS automatically by default, have a custom theme and do many other fancy things.

If you have no idea on how to do all of this, I'd recommend against doing it. Also remember that it's old hardware we are dealing with, so compatibility might not be the best, if existent at all.