Assistance creating hybrid MBR/GPT on High Sierra to allow Windows to install

Hi,

Would really appreciate some help trying to install Windows 10 as dual boot on my late-2009 (identifier iMac 10,1) 21.5 inch iMac. The issue I believe is that Windows needs a hybrid MBR/GPT on my iMac model, and as of High Sierra this hybrid is no longer created.

Years ago I was dual-booting Snow Leopard and Windows 7, upgraded both OSes over the years and have got an aftermarket SSD (Samsung EVO 850 1TB). Up until a few months ago I was happily dual-booting Sierra and Windows 10 Creator’s Edition (1703) fine, then had an issue with the Windows side (?due to Fall Creator’s Update 1709) where it would fail to boot.

I’ve now clean-installed High Sierra as I’ve upgraded every time since Snow Leopard and thought I might as well spring clean and start fresh. This converted my Mac booting partition to APFS and High Sierra is working fine.

My partitions at present are (in order): Macintosh HD (APFS, boot partition), ‘Bootcamp’ (FAT32, will be converted to NTFS by Windows installer) and Mac Data (APFS, for all my media/documents etc). Note: these are partitions I’ve created myself, not via Bootcamp Assistant, as that will not allow me to use my Windows 10 installation DVD as it says only Windows 7 is supported on my iMAc

Ideally I’d have a big ExFat partition too that both High Sierra and Windows 10 would be able to read & write too, but Windows will only see the first 4 partitions due to requiring MBR, and the first 4 are taken up by an EFI partition, Macintosh HD, Mac recovery partition and the Windows Bootcamp partition itself. I can’t think of a way around this unfortunately.

So, the issue itself: from reading around, it appears that as of High Sierra, Disk Utility no longer creates a hybrid MBR/GPT when creating a FAT32 partition, as it always used to. Thus my SSD just has a GPT, and Windows refuses to install on a GPT disk on this iMac (I believe due to an old/incomplete version of EFI on this late 2009 model).

Therefore, I think I need to somehow create this hybrid MBR/GPT to get Windows to successfully install and boot. I’ve read these articles https://discussions.apple.com/thread/8100495 and https://apple.stackexchange.com/questions/308824/how-to-convert-a-drive-from-the -gpt-format-to-the-hybrid-gpt-mbr-format-when-usi but it’s a little bit complicated. I can understand bits of it but some of it’s a bit over my head!

Would anyone be able to break the instructions down a bit more and give me a step-by-step method of how to create the hybrid MBR/GPT?

The output of diskutil list disk0 in Terminal is shown below:

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_APFS Container disk2 109.8 GB disk0s2

3: Microsoft Basic Data BOOTCAMP 135.2 GB disk0s3

4: Apple_Boot 134.2 MB disk0s4

5: Apple_APFS Container disk1 754.9 GB disk0s5

And below is the output ofsudo fdisk /dev/disk0 (after disabling SIP) which I believe shows the lack of an MBR table??:

Disk: /dev/disk0 geometry: 121601/255/63 [1953525168 sectors]

Signature: 0xAA55

Starting Ending

#: id cyl hd sec - cyl hd sec [ start - size]

------------------------------------------------------------------------

1: EE 1023 254 63 - 1023 254 63 [ 1 - 1953525167] <Unknown ID>

2: 00 0 0 0 - 0 0 0 [ 0 - 0] unused

3: 00 0 0 0 - 0 0 0 [ 0 - 0] unused

4: 00 0 0 0 - 0 0 0 [ 0 - 0] unused

Very grateful for any assistance!