Will not boot after 10.10.2 security update

I downloaded the security update as usual. computer starts to boot back up, gray screen with Apple logo & progress bar gets 2/3 & stops.

I restarted, used the disc utility to check & repair errors. nothing found. Re-installed os, but nothing works.

i know this would be a pain, but if I remove the internal drive. Place a new on inside the machine & put the old one in an external enclosure. Install os on computer & drag old files from external drive. Would this work?

thanks,

matt