OSX hangs on startup after large file search caused a crash (Mavericks 10.9.5)
It boots! I finally found a thread of others with identical symptoms: Gray screen on normal boot, blue screen with lines even when booting to recovery, crazy fan spinning. 

It was my GPU.

Standard problem for this model of macbook, and Apple willfully ignores it. 

Instructions on how to fix it can be found in this thread.

I'm reposting them here for the sake of making this information as widely available as possible.

Disable your AMD graphics card by doing the following (it'll force you to use your intel graphics card instead -- not powerful, but functional).

/sbin/fsck -fy /
/sbin/mount -uw /
mkdir /Disabled_System_Library_Extensions
cd /Disabled_System_Library_Extensions
mv /System/Library/Extensions/ATI* .
mv /System/Library/Extensions/AMD* .
touch /System/Library/Extensions
exit


Reboot and your computer will startup as normal.