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You need to backup all user files and information out of later proprietary file formats, for instance right click on iPhoto Library and copy out the Originals folder for later importation into the older version.

Same thing with iTunes music or anything else saved in later program versions of the files. Like text and basic graphics formats etc. It's because older verisons of programs alter their files that can't be read by older versions sometimes.

If you want to make double sure, first Carbon Copy Clone your 10.8 drive to a extenral drive first, that way you can later access it and even option key boot from it to get data out and back to Snow Leopard.

Most commonly used backup methods

Save everything to a external storage drive (not Timemachine or backup program) then disconnect or use the 10.8 bootable clone as mentioned above.

Then hold c or option key boot (wired or built in keyboard only) off the 10.6 install disks that came with the machine and use Disk Utility to erase the ENTIRE drive (select the drive make and name) use the Securirty Option > Zero All Data if your allowed, when finished then quit and install Snow Leopard, setup and update to 10.6.8.

How to revert your Mac to Snow Leopard

How to erase and install Snow Leopard 10.6

For Snow Leopard Speed Freaks