I've had this problem happen on two computers (Win7x64) on which I'm running GRUB; one of those is at work, and one of those is at home. In my case, though, I only have one hard drive.
Although I don't object to backing up my GRUB MBR, installing a W7 bootloader, and then restoring my GRUB MBR later after I've successfully installed KB3033929... to hell with that. I'll just disable the update, and let it install later on once everything is fixed, unless Micro$oft wants to reimburse me for my labor at the $50/hr rate that my company thinks I'm worth.
Hey, Microsoft, there are plenty of us out here who multi-boot! Why would you possibly think it's cool to create an update that refuses to function unless we're running a 100% Microsoft system?! What does the MBR - WHICH EXISTS OUTSIDE OF MY WINDOWS PARTITION
- have anything to do with an update that takes place *within* my Windows partition?!