Decoding Your installation options
It’s easy to miss the Mac OS X installer screen’s Options item. Clicking on this small button reveals several significant choices for how OS X gets installed on your Mac. Here’s what each method does.
Upgrade Mac OS X This option keeps your older version of Mac OS X, but installs new versions of any files that have changed between the older version and Leopard. Although in theory this should be safe for everyone, third-party system add-ons, as well as damaged system files that aren’t replaced, can cause problems. I recommend it only for brand-new Macs being upgraded from Tiger to Leopard out-of-the-box. For most users, I recommend Archive and Install (or Erase and Install, restoring your data from a full backup using Setup Assistant).
Archive and Install This method installs a complete, new version of Leopard, using none of your older OS installation files. However, it keeps all of your older system files on your hard drive—in a folder called Previous System 1 (or 2 or 3, depending on how many Archive and Install installations you’ve done)—just in case you need any of those files. This method avoids many problems because the contents of your old System folder, including some third-party system add-ons, are archived in the Previous System folder. I recommend choosing the option to Preserve Users And Network Settings, because this retains your existing user accounts and all user data in Leopard. (Note that the Archive and Install option requires much more hard-drive space than an Upgrade install. It may also require a bit of manual file transferring, as described on the previous page.)
Erase and Install This method actually erases your entire hard drive—or, if your drive is partitioned, the partition you’ve chosen as the install location—and installs Leopard. (It should go without saying that you must have a backup of your data to use this option, since erasing the drive deletes all your data.) This is useful if your current OS X installation has been having problems, as it guarantees that any disc or file corruption is eliminated. (Choose the option to Format Disk As Mac OS Extended [Journaled].) However, unless you really want to start from scratch, after the installation you’ll need to either recreate your user accounts in Leopard and restore your data from a backup, or use OS X’s Setup Assistant to transfer those accounts and data over from another computer or from a full backup. (This latter procedure nearly replicates an Archive and Install with the Preserve Users and Network Settings option—but with the added bonus of a new, clean hard drive.)
Install Mac OS X You’ll see this option— instead of Upgrade Mac OS X—only if you choose a volume that doesn’t already have an upgradeable version of Mac OS X installed. In that case, Install Mac OS X and Erase and Install will be your only choices. (If you received your copy of Leopard through the Mac OS X Up-To-Date program—for people who purchased a Mac that shipped with Tiger after Leopard was released—your disc will install only onto a drive that already has Tiger installed. You’ll get the same installation options except for this one—you won’t be able to install Leopard on a non-Tiger drive.)
[ Senior Editor Dan Frakes has installed Leopard more times than he cares to admit. ]
This article was updated at 5:30 p.m. on October 27 to clarify information about the Erase and Install method and to include more inforation on system requirements.