Disable mounting Internal Macintosh HD at boot [duplicate]

My iMac (mid-2010 model, Snow Leopard) has a problem with S.M.A.R.T status: Failing and it has become read-only. Because I didn't want to waste money on the repair, I bought an external HD, so the internal HD is now unnecessary. How can I disable mounting the 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:

1. 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
2. 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)
3. add a line like this one: UUID=F0E430C1-5558-3BB3-9FA9-6904B663FEEA none hfs rw,no auto of course replacing the uuid with yours.

I'm not sure if it works with external drives, but the "normal" way to change your start drive is this:

http://www.dummies.com/how-to/content/how-to-change-mac-snow-leopards-startup-disk-prefe.html