OSX Yosemite upgrade error: “a mismatch between the MBR and GPT partition maps…”

Question

My MBP's HDD is half OSX and the other half is Bootcamp.

While booted into OSX I decided to let Software Update install Yosemite. I left the room. Upon returning a while later I found the following error message: "a mismatch between the MBR and GPT partition maps is not supported".

I found an excellent post in this forum by @klanomath that dealt with a MBP stuck in an endless upgrade attempt loop like I'm experiencing, but that situation required reinstalling OSX; I'm hoping this "mismatch" can be repaired so the installation can continue.

If not, is it possible to interrupt the endless installation attempts and see if the existing OSX is still bootable? (has the existing OSX installation been destroyed by the upgrade yet?)

I found another post in this form, by @AndyD273, where it looks like a proposed solution exists but I do not know how to get from this endless loop state into the position where I can download, much less install, gdisk and use Terminal (and I cannot ask via comment in that post because I lack the "reputation"). All I know how to do is use the Option key to get to my Windows 8.1 partition.

Answer 1

To interrupt the endless installation attempts you have to remove the temporary Yosemite installer folder and some invisible files in the root of your main volume.

Boot to Recovery HD by pressing cmdR right after the start-up chime.

Open Terminal from the menubar Utilities -> Terminal. cd to your main volume in the /Volumes folder with:

cd /Volumes/name_of_main_volume

Get an overview with ls:

ls -la

Then remove all Yosemite installer related files and folders:

rm -R OS\ X\ Install\ Data/
rm .IAProductInfo
rm .OSInstallMessages
rm .OSInstallerMessages

Enter exit to quit Terminal.

Now open "Choose startup disk", choose your main volume to "re-bless" it again and reboot your Mac.

Download and install gdisk and continue repairing the MBR/GPT partition table like described in the answer here.

Answer 2

@klanomath's answer got my system out of the Yosemite upgrade loop, and the link he provided led to fixing the mismatch between the GPT and MBR partitions.

BUT...

The subsequent Yosemite upgrade installation did something to corrupt booting the formerly usable Bootcamp installation. This partition still exists and has the original contents but it is not bootable.

If this were a Windows laptop there'd be several tools available to fix this boot corruption. But I know nothing about fixing this in the Mac/OSX realm. Forum posts like this, this, and especially this might contain a fix but over the several-page span of each thread the "answers" appear to change and ultimately not even work for the OP and/or others. My reading also suggests that it's common for Yosemite upgrades to corrupt a Bootcamp installation. If that's the case surely there's a tool or single consistent slam dunk solution for correcting this problem.