Hello and welcome to my User Tip
You need to erase and install 10.6 Snow Leopard, I will show you how to do this.
If your here because you upgraded to 10.7 Lion and want to go back to 10.6 Snow Leopard you need to see this thread.
A little more preparation is in order to make sure your files work in Snow Leopard.
If your here because you have a problem and you think erasing and reinstalling is going to solve it, it most certainly will, but you might not need to go this far and you might want to recover data off the non-booting drive first.
If your here because you want to secure erase data, this tip doesn't cover that, see this one then come back here
Ready to erase and install Snow Leopard?
Your data will be gone after this and not recoverable what so ever, don't come crying later that you missed something.
You might want to take this chance to export web browser bookmarks, grab product keys or even clone the old 10.6 drive before you erase it.
Unmount and disconnect all other drives. No mistakes. Here we go.
Hold c (or option key) boot off the 10.6 installer disk (or hold option key boot off the 10.6 clone drive) and under the Utilities menu (folder) is Disk Utility.
Select the entire internal boot drive on the far left, it will have the drive makers name and size.
Do not select the indented names, those are just partitions on the drive, you need to erase the entire drive to catch everything.
Click Erase > Security option > Zero All Data (will map off any failing sectors) go watch a movie, it takes a bit and improves your hard drives reliability and read speeds. (SSD no need zero, just erase). Trust me this works wonders!
If the Disk Utility > Zero Erase fails or hangs, the drive has run out of spare sectors and is worthless, it will have to be replaced, better you found out now than later with your data on it.
Check under Partition: Options that you have a GUID and Format: OS X Extended Journaled, if not change it to those and apply.
Quit Disk Utility. You will be back into the 10.6 installer.
Fresh installing Snow Leopard 10.6
If your using your option key bootable 10.6 clone, use the cloning software to reverse clone 10.6 onto the internal drive then return your files from the storage drive. Your done, see you later.
Now for you who just erased the internal drive via the 10.6 disk and need to install fresh.
"Fresh" means no possibly corrupt TimeMachine, Migration Assistant, or clone restores. New OS X, programs and only vetted files returned.
Performance type swear by this method as it allows the leaving of previous junk, malware, old files etc behind. Trim and speedy.
1: Quit Disk Utility and install 10.6. from the installer disk.
Reboot, setup and use the same user name as before. (different password ok) Why?, because you can place your user files from backup right on in and your iTunes playlists, permissions etc work. Unless you don't want to use the same name, then you lose the playlists and have to recreate them, change the permissions for files.
2: Use Software Update to get up to 10.6.8 fully, repeat until clear.. This is very important to do or you'll have problems later.
3: Install all programs from fresh original sources and compatible with 10.6.8
If by chance you get a hang upon reboot after installing third party software, hold the shift key down.
If you had 10.5 previously on the machine, you'll notice your iLife is missing with the 10.6 install. This is because the 10.6.3 disk doesn't have it, only the machine specific disks do that come with the Mac when it was new.
You can either buy the 10.6 iLife from Apple or use Pacifist from CharlesSoft to extract the 10.5 versions from the 10.5 install disks. You paid for it so it's yours.
4: Once you have all your third party software installed (as best as you can of course) next is to hook up your storage drive.
Return the contents of those user files folders: Music, Pictures, Movies, Documents etc (not Library) to their respective folders in each account
Hard drive performance tip
If you installed exactly in the order as I have outlined above (1, 2,3,4), and keep your boot drive below 50% filled, it will operate at peak performance forever until it dies, or you damage it by moving it around while it's operating.
Your hard drive performance will suffer greatly the more OS X boot partition data is written on the second 50% of the drive, even if you remove some to get under 50%.
If there is any data on the second 50% of the drive, the heads have to travel there to get it, use the smaller sectors and that slows read/write performance on hard drives (SSD no need to worry).
Whatever you do, don't go past 80% filled on boot drives on Mac's as your pressing your performance and stability luck.
That's it, enjoy.