"the window of opportunity for Windows 8 to detect a request for a change to the OS’s boot order is just 200 miliseconds. That’s just 200 miliseconds to slap the F8 key if you want to start your system in Safe Mode or boot from another source.
To deal with the issue, Microsoft has developed a three-tier solution that includes a new boot options menu accessible from within the operating system after start up is complete, a fail safe that automatically brings you to the boot menu if Windows doesn’t start properly and a way to get to the fail safe boot menu even if nothing is wrong with your computer