We’ll divide this article into two sections. The first focuses on reverting to Snow Leopard from Lion and removing Lion completely. The second demonstrates how to install Snow Leopard on a second drive or partition so that both operating systems can coexist peacefully.
Reverting to Snow Leopard
These methods get rid of Lion altogether and bring your Mac back to Snow Leopard. It’s important to note that this will only work on Macs that are Snow Leopard compatible; this excludes Macs released with or after Lion, as Snow Leopard’s kernel doesn’t support this new hardware.
The first option is to restore from Time Machine. Mac OS X allows you to take an existing Snow Leopard Time Machine Backup and restore a Lion-based Mac back to Snow Leopard (thanks to the fine folks over at Mac|Life for highlighting this method).
First, boot into Lion’s Recovery Drive by holding down the R key at startup or holding down Option and selecting the Recovery HD. This will launch Lion’s specialized recovery partition which is intended to help users troubleshoot and reinstall Lion. We can use it, however, to restore Snow Leopard.
Choose to Restore Your Time Machine Backup in Lion’s Recovery Partition to Begin the Snow Leopard Restoration
With your Snow Leopard Time Machine drive connected to the Mac, choose the option to “Restore From Time Machine Backup.” This will commence the Time Machine restore process and, when completed, your Mac will boot back into Snow Leopard, current as of the date of your last backup.
Your second option is to wipe out the drive and perform a fresh install of Snow Leopard. To perform this step, first make a manual backup of any data you wish to preserve. This is important because this option will erase all data on your system drive. Next, boot to either the Lion Recovery Partition or the original Snow Leopard installation disc if you have one. In either instance, launch Disk Utility and select your internal hard drive partition from the sidebar on the left. Choose the “Erase” Tab on the right, set your Format to “Mac OS Extended (Journaled),” give the drive a name (i.e., “Macintosh HD”), and press “Erase” on the bottom right. After a minute or two the drive will be completely erased and ready for a clean install of Snow Leopard from either your Mac’s original system discs or a retail copy.
Running Both Lion and Snow Leopard
This second option is my personal preference. Lion may not be perfect now, but it’s the future for Apple and so it’s important to become accustomed to its new features and UI elements.
The best way to approach this situation is simply to keep both operating systems around. You can do this two ways: either add an additional partition to your internal hard drive for Snow Leopard, or install Snow Leopard to a separate disk (either internal in the case of Macs with multiple internal drives or external for all Macs via Thunderbolt, FireWire, or USB drives). Thunderbolt or FireWire drives are preferred for their relative speed, but for occasional tasks in Snow Leopard even USB 2.0 will suffice.
First: partitioning. For those with Macs that only have one internal drive bay, or those who want to take Snow Leopard with them but don’t want to carry an external drive, partitioning the system drive is the way to go.
First, quit all applications on your Mac and then launch Disk Utility. Select your drive from the sidebar on the left (note: you want to select the drive – the entry with the capacity listed – not the volume — See the screenshot below for an example). With the drive selected, choose the “Partition” tab on the right. Press the plus button on the lower left to add a second partition to the drive.
By default, this second partition will take up all the free space left on your first, and current, partition. Resolve this by dragging the slider between the partitions until you’ve found a good mix between the free space on partition one and the overall size of partition two. Apple requires 5GB of free space for Snow Leopard, but I’d recommend giving it a bit more – at least 15GB.
Don’t worry too much about file and media storage, as you’ll be able to access files from either partition and can store all your media on partition one yet still be able to access it from partition two.
Use Disk Utility to create a second partition for Snow Leoaprd on your primary drive.
With your partition sizes set, name the second partition and press “Apply” in the lower right. This will dynamically resize your current partition and create the second partition for Snow Leopard. Next, get your Snow Leopard installation media and boot to it by holding down the C key at startup or the Option key and then choosing the disc from the startup manager. Run through the installation process as normal, and when you are prompted to choose the installation drive, choose your newly-created partition. Once the installation is complete, you can boot into Snow Leopard by holding down Option and using the boot menu.
The second option for running both operating systems is to install Snow Leopard to a second drive. This is even easier than partitioning as all you have to do is install a second internal drive or connect an external drive, launch the Snow Leopard Install Disc, and then run through the installer process as normal. Just as with partitioning, when the time comes to select an installation disk, choose your external drive and allow the installation to finish. Now you can access Snow Leopard with the boot menu any time the internal or external drive is connected.
With any of these options, make sure to boot into Snow Leopard initially and run Software Update. No Snow Leopard installation media will provide you with the current version of the operating system and it is important for security and performance reasons to upgrade to the most recent build.
It’s a near-certainty that Lion will eventually reach a point that is acceptable to all users and their software but, until then, these methods can at least help you get by, either via reverting to Snow Leopard completely or by keeping a bootable Snow Leopard drive or partition on hand.