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Urgent! I have an existing Windows 10 installation via Bootcamp which has been on my Mac for months without issue. However for ages I have been wanting to read my Mac partition from Windows which it doesn't allow me to do, so I looked online which suggested making a third partition for both OS's to read from.

So I made a 10GB partition on Disk Utility which worked fine, and my Bootcamp drive is still there (it wasn't formatted, I can still view all Windows files from OS X), only problem now is Windows will not boot! If I hold down alt at the white screen, only my Mac partition shows up. When I go to system preferences and choose boot options, bootcamp is still there!? I removed the 10GB partition but it has not remedied the problem... I don't get it, please help!!

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I can help you if you've a live USB linux distribution, ex. Ubuntu.

  1. Boot into Ubuntu live USB by pressing alt when upon restarting and choose EFI Boot.
  2. Choose to Try Ubuntu.
  3. Open Terminal by ctrl + alt + T.
  4. Install efibootmgr by entering sudo apt-get install efibootmgr.
  5. After installing it, check bootOrder and boot values by entering sudo efibootmgr.
  6. Usually you OS X boot value is 0x80, please get your Windows value.
  7. You can change booting order by sudo efibootmgr -o <comma separated boot values>.
  8. Adjust your bootOrder assuming your Windows boot values is 0x01 then you should enter sudo efibootmgr -o 1,80 <- Windows first. So that your Mac should try booting to Windows first as you mention that you need an urgent solution.
  9. Reboot to your live USB again and readjust the bootOrder by letting Mac boot into OS X first by sudo efibootmgr -o 80,1 <- OS X first.
  10. Don't use any 3rd party bootloaders as you can easily adjust everything manually.

Advice:
Format your shared partition as ExFAT =)

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