Apple's Snow Leopard OS: Ultimate Upgrade FAQ
In-place Upgrade vs. Clean Install
With a major OS release, there's always the question of whether to perform a simple upgrade (where your files, applications, and system configuration files are left in place) or a clean install (where the target drive is backed up and erased before installation).
In the past, I, like many power users, advocated for a clean install -- it can help avoid conflicts between older applications and the new OS or any damaged configuration files. It can be a good troubleshooting step if you can't find the source of ongoing problems or if you have damaged system files. But the snazzy new installer for Snow Leopard (more on that in a minute) detects potential issues, making a simple in-place upgrade not only sufficient, but the better and easier way to go.
The main advantage to a clean install is from a personal housekeeping perspective. Since most computers over time collect files that are no longer needed -- anything from old to-do lists to applications you installed but never use -- a clean install forces you to take a look at what's on your computer and do some spring cleaning. Given that it means copying files, erasing the disk, restoring and organizing your files, and resetting all of your preferences and system settings, a full-fledged clean installation it isn't worth the hassle. Just find some time to go through your hard drive and clean it out.
If you do perform a clean install, you'll need to boot your computer using the Install DVD by restarting the computer while holding down the C key. Then use Disk Utility (located in the Utilities menu in the menu bar after you've booted from the Install DVD) to select your hard drive and erase it. Quit Disk Utility to return to the installer and proceed with your installation as outlined below. Be forewarned: Starting up from the installation disc and erasing your hard drive for a clean install will make the entire installation process take longer.
Longtime Mac users are probably familiar with the old "Erase and Install" (which erased the contents of the hard drive before installing the new OS) and "Archive and Install" (which retained a copy of all system files for later retrieval) options for installing Mac OS X. Although you can still erase your hard drive for a clean install using the steps outlined above, these two options are no longer part of the actual installation process.
Meet the Mac's New Installer
When you put the Snow Leopard Install DVD in your Mac, the Finder window that opens looks pretty much like that of any Mac OS X installation disc (the obvious exception being the Snow Leopard icon).
But the second you double-click the installer, you'll notice something's different. Instead of being asked to restart your Mac and boot from the Install disc, you'll see a screen with two options: Utilities and Continue. That's because you don't need to boot from the DVD to install Snow Leopard (though you can do so later down the road if your Mac has problems or if you want to erase your hard drive before selling your computer, for instance).
The reason your Mac doesn't need to boot from the Install DVD is because one of the first things the installer does is copy all the files you'll need for installation to your hard drive. This has a few important benefits. First, it saves you time because you don't have to wait for your Mac to boot from the DVD (which often takes a few minutes or longer), and the installation itself goes more quickly because the files are being installed from your hard drive.
It also means that when the Mac checks the integrity of the files to be installed (which used to mean verifying the integrity of the DVD, a process that took so long most people opted to cancel it), it does so after the files have been copied to your hard drive, speeding up the process but also making integrity checks a built-in part of the installation (since you can't skip them).
The Snow Leopard installer has a couple of other tricks up its sleeve. Before installation, it scans your Mac for any applications or other tools that extend Mac OS X (like third-party System Preferences panes or device drivers) that are known to cause problems with Snow Leopard. The installer doesn't delete them, but it does move them out of any system folders or directories to ensure a smooth installation.
The installer also has a safe redo feature. If something interrupts the process (say you unplug your iMac by mistake), that's not a problem for Snow Leopard. With the installation files actually on your Mac's hard drive and the fact that every part of the install is written to a log file, the installer can simply pick up where it left off when your Mac is restarted.
Now, back to that initial installation screen: The Utilities option lets you restart and boot the computer from the Install DVD (the same as if you held down the C key during a restart) in case you want to erase or repair your hard drive, or restore it from a Time Machine backup. If you're doing a simple upgrade to Snow Leopard, click the Continue button.
In addition to skipping the initial restart used in earlier installations, Apple has pared down the number of clicks it takes to install Snow Leopard. After you click Continue, the installer automatically selects your startup drive (the one that your Mac booted from). Many people only have one internal hard drive or partition, but if you have multiple drives or partitions, you can choose another drive; just click the Show All Disks button and make your selection.
Unless you want to customize the installation (more on that in a moment), click the Install button now. You'll be asked to confirm that you want to proceed and asked to provide your admin username and password. After that, take a break; there's no more user interaction required. (The computer will restart itself during the installation process without any input from you.)
The whole installation process typically takes 30 to 40 minutes, although it varies depending on your hardware and your installation choices -- I've seen some installations finish in notably shorter and longer time spans.
When you come back, your Mac will be running Snow Leopard. When you first log in using the new OS, the Setup Assistant plays the standard Mac OS X welcome video. If you've done an upgrade rather than a clean install, there is nothing to set up and you can close the Setup Assistant window after the movie plays. If you did a clean install, you'll need to provide registration details, and then the Setup Assistant will guide you through the basic setup and configuration steps. You're on your way.