Mac OSX Snow Leopard Upgrade: The Facts
Everything you need to know for an easy install
By Ryan Faas | Computerworld US | Published: 12:54, 28 August 2009
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.