This was in another thread, but now I can't find it. You need to grab the ACHI kext from 10.8. Fortunately for us, 10.8.1 actually makes a backup, so this is what you can do:
Start up in safe mode (-x to Chameleon)
Open up the Terminal (from cd to MacOS/ is one line).
Code:
cd /System/Library/Extensions/IOAHCIFamily.kext/Contents/PlugIns/IOAHCIBlockStorage.kext/Contents/MacOS/
sudo cp IOAHCIBlockStorage.hpinst.orig IOAHCIBlockStorage
sudo touch /System/Library/Extensions
Then just rerun the ProBook Installer and install the AHCI patch. I'm not entirely sure if this step is necessary, but it doesn't seem to harm anything and my ProBook started up regularly after I reboot.
Edit:
Just to give original credit (Post 648):
http://www.tonymacx86.com/hp-probook/63348-hp-probook-installer-4-1b1-65.html#post428441