Quote:
Originally Posted by dissentowner
Well you can't go back to SL, Lion won't let it install. 
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If you insert your SL DVD, and reboot your computer while holding down C (for cdrom, a bit of an anachronism), that will let you boot off the SL installer disc.
From there, ignore the installer, instead go to the menu bar and find the selection to launch the Disk Utility. Once in there, you'll want to reformat the drive with a new empty HFS+ journaled non-case sensitive partition. You can then exit disk util and proceed to the SL installer and do your reinstall.
Doing it this way will wipe the disk though - you'll need all your data backed up before doing any of this.
I wouldn't recommend doing it any other way though - Lion's HFS filesystem implementation could be a little different under the covers to support full disk encryption and snapshots, so I wouldn't want to risk disk corruption trying to install SL and having something horrible happen down the road.