Disable mounting Internal Macintosh HD at boot
This is quite old, but still works perfectly under Lion, I'm using it for several partitions: http://hints.macworld.com/article.php?story=20060930150059172

Fast path:


find the Macintosh HD volume UUID: while your Macintosh HD partition is mounted, open Terminal.app and type diskutil info /Volumes/Macintosh\ HD, then read the line beginning with Volume UUID
edit /etc/fstab as root (I personally use vi but any editor will do, the article even mentions pico! If you have TextWrangler installed - you should - with the command line helpers, sudo edit /etc/fstab will work too)
add a line like this one: UUID=F0E430C1-5558-3BB3-9FA9-6904B663FEEA  none  hfs  rw,no auto of course replacing the uuid with yours.