Your new Snow Leopard DVD will install on your current Leopard system, updating all the system files and preserving your user data, preferences and Library. Except when something goes wrong. Here's a guide on how to wisely prepare for your Snow Leopard install.
The Very First Questions
The first thing you should ask yourself is, "Do I need to do this right now?" For example, do you (or someone else in your family) have a big project looming that requires your system to be 100 percent available? Would an upgrade, and possible difficulties, interfere? Next, do you have enough external disk storage for a backup? You may need to go shopping or place an Internet order first. We'll discuss that in a moment. Are you planning a trip? Would difficulties and possibly being cut off from the ability to print boarding passes or check hotel reservations become a problem? Your enthusiasm to get on with the cool Snow Leopard upgrade should be tempered by these considerations.
Okay, so now you're ready to go get the Snow Leopard package. Remember, if you're going to install it on multiple family Macs, you're obligated to buy the Family Pack for $49.
The next step is to look around and find the master DVD that shipped with your Mac. That's a Mac OS X DVD that's guaranteed to boot your particular Mac, and it might come in handy if something goes wrong.
Finally, use /Applications/Utilities/Disk Utility to verify your internal hard disk. The last thing you want to do is to embark on an upgrade with a corrupted or damaged internal disk.
Access to Storage is Paramount
Nowadays, we work with gigabytes of data: photos, personal videos, iTunes music, TV shows and movies, and other personal work. We can do that because rotating disk storage is so cheap. So if you're one of those people who owns a Mac but owns no other external drive(s), now is the time to change that. One or two extra, external hard disks is just about essential for the modern Mac user. After all, an Otherworld Computing, terabyte, quad-interface external drive costs as little as US$165, and the peace of mind and flexibility it provides is enormous.
Terabyte drives in 2009 are fairly inexpensive
Time Machine is Your Friend and Savior
The next step is to use one of these external drives as your primary backup, preferably using FireWire 800 but USB 2 if you must. Don't plan on using that drive for any other storage.
Perhaps the most important feature of Leopard was the introduction of the Time Machine backup software. Not only does it allow you to go back in time to recover a deleted file, but for our purposes here, it allows you to do a complete system restore from a time-stamped Time Machine backup. That ability to do a complete* system restore is essential if something goes wrong with the Snow Leopard upgrade process.
I recommend your Time Machine drive be two or three times the capacity of your internal drive. That's because you might end up buying a third drive for overflow storage, say an external iTunes library, and you'll want to be able to back up that drive as well. Time Machine allows you to back up multiple drives to one large drive.
So, right now, before you do anything else, make sure you have an external hard disk that's large enough and get started with Time Machine -- if you haven't already. Here's a good starter tutorial.
Finally, before you start the upgrade, check the weather forecast. Pick a sunny weekend morning that's open ended. It's been a stormy summer all across the U.S., and the last thing you want is to be in the middle of a major upgrade and have a storm move in with lightning crackling all around.
If Something Goes Wrong
It's very likely that your Snow Leopard Upgrade will go swimmingly and you'll be up and running in under an hour. However, there's a small chance that something could hiccup. Your Mac may not boot up right, files could be missing, or the drive may have become corrupted. There are many things that might happen, and a complete list of remedies would be outside the scope of this article. Let's assume here that you've used Disk Utilities to check your hard disk, and it seems okay. But you've decided you want to restore your Mac to the way it was before the upgrade and regroup.
That's where your Time Machine backup comes into play, and you'll be using a feature called "Restore System from Backup." The canonical article on restoring a system from backup was written by James Davidson and has plenty of screen shots to guide you. In fact, that might be a good article to actually waste a little paper on and print a copy before you start the upgrade.
Once your Mac is restored to exactly the way it was before you upgraded, you'll have the liberty to rethink the situation and do some research on the Internet to see if anyone else had similar problems. You may have had a funky USB device attached that threw the installer for a loop, so it's always best to do the upgrade with a wired keyboard and mouse and no USB devices attached. If you go for a few hours or days before proceeding, let Time Machine continue to conduct its backups so you'll always be up-to-date.
In summary, a little bit preparation will help you prepare for a worst case scenario and insure that you have an archival copy of your Mac in its current Leopard state.
* Caches plus Spotlight and mail databases aren't saved, so when the restored system boots up, there may some housekeeping required, that is, automatic rebuild of these databases. That may take a little time and slow the Mac down for awhile. Here's a list of exclusions.