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