I'm having the exact same issue, I have a late 2007 white macbook with Lion Compatible hardware. I planned on installing lion to an external hard drive to launch specific software and I didn't want to lose the ability to use snow leopard. The lion installer booted just fine but then when it came to booting up from the external usb drive that I installed lion onto, my macbook wouldn't recognize the disk. Strangely on another core 2 duo machine (iMac late 2009) the hard drive was recognized and I am typing this as we speak on this external drive.