Sigh. Its been one of those days where every box I touch has blown up spectacularly and going through this taught me some interesting things that I didn’t know (or know specifically how to do) so here are some tips when your Mac won’t boot:

Command+R gets you to the low level tools for re-installing the OS. Hold Command+R down until you see the Apple logo. This should work but doesn’t always. Command+V held down until you see the Apple logo gives you a BSD style unix text listing which tells you how the boot process is going. sudo nvram boot-args="-v” entered into a terminal window (type it and then press enter) makes the text mode boot process the standard and I’ve now done that on all my machines Command+S held down until you see the Apple logo boots the machine in single user mode. At that point you can mount the hard drive as read/write to make changes. If you see the message: BootCacheControl: Unable to open /var/db/BootCache.playlist: 2 No such file or directory then chant “iFixit Rocks!!!” and read this article and follow the directions.