OSX Yosemite upgrade error: "a mismatch between the MBR and GPT partition maps..." 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.
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.