I have updated MP to 1.2.57 and tried everything to make the latest firmware load (I click on beta versions and wait till the icons change). same thing each time after it says 'done' 2 motors on my hex randomly spin and I cannot connect to the my APM 2.5. either via usb or radio link., I roll back to 2.9.1b and its fine no problems. I do my calibrations then I go off and fly no worries everything is great!
I have tried many times am I missing something vital?
Uninstall Mission planner from your computer.
Reboot and then install the latest version.
I had this same problem with update from within mission planner, doing a clean install worked for me.