Q: Clean install of Snow Leopard from Mountain Lion.

once your erase and format it to MAC OSX extended (journaled) exit disk utility and run the installer from the disk.

so you need to boot into the disk and run the installer.

What is the external drive for?

Once you erased the HD, you can just restart from the SL CD and let the installation proceed.

I usually just erase the partition and make sure it is Mac OSX Journalled or whatever and that GUID Partition Table is selected under uptions. I'm using the utility right now so I can't check what the actual wording is.

Afterwards, close disc utility and you should be right back to your installation page. Most of it's fairly straight forward from there

ETA: guess I typed too slow. I've basically repeated what someone else said.

Are you sure you have backups? Are you sure you're selecting the right disc to erase?

Then, Yes.

Linkblock wrote:

 

So in disk utility I highlight my HD and press erase?

Yes.

just unplug the external !!! until you need it

A guide here:

http://support.apple.com/kb/HT3910?viewlocale=en_US

TheSmokeMonster wrote:

 

just unplug the external !!! until you need it

Yes. I completely forgot to bring that up. Make sure you don't have any peripherals plugged in while your re-installing. This can cause problems. Only keep what you need to operate your Mac plugged in (keyboard, mouse, etc...)

Best of luck to you.