Specifically, the procedure involves erasing your Mac's drive, installing Mavericks onto it, and then importing all your data from your backup. (If this sounds a lot like a clean install, that's because it's essentially the same process.) Here are the steps to take:
- Make sure you have an up-to-date backup--either a Time Machine backup or a clone backup using a utility such as SuperDuper or Carbon Copy Cloner--of your Leopard Mac's drive. (For this purpose, I recommend a clone.) Be sure to test this backup to verify that it has your latest data: In the case of a Time Machine backup, try restoring some important data from the backup; in the case of a clone backup, boot from the clone to make sure it boots and that it contains all your data.
- Use the computer running Snow Leopard or later to download the Mountain Lion installer from the Mac App Store. (If you've already got your copy of the Mavericks installer, skip this step.)
- Create a bootable install drive using the instructions for creating a bootable Mavericks install drive.
- Boot your Leopard Mac from that new install drive. When you do so, you see the initial Install OS X screen.
- From the Utilities menu at the top of the screen, choose Disk Utility.
- Use Disk Utility to erase your Leopard Mac's internal drive. To do so, select that drive on the left, click Erase on the right, choose Mac OS X Extended (Journaled) from the Format pop-up menu, and click Erase. Warning: This step erases all the data on your Mac's drive, which is why you needed that backup!
- When the erase procedure is finished, quit Disk Utility to get back to the Install OS X screen.
- Click Continue to install Mavericks on your Mac's internal drive.
- After your Mac restarts, installation finishes, and you proceed through the setup process, watch for the Transfer Information To This Mac screen. You'll choose either the option to transfer data from a Time Machine backup or to transfer data from another startup drive (such as a bootable clone backup), depending your backup type. This step transfers all your files from your backup to your new installation of Mavericks.
When the transfer process is finished, you'll be able to log in to Mavericks with all your accounts and data intact.
The quick-but-techie way
If you're comfortable diving into the OS and editing a .plist file, this is the fastest way to install Mavericks over Leopard, although, as with the previous method, you'll need to be able to boot from a Snow Leopard, Lion, Mountain Lion, or Mavericks drive to actually run the installer.
As I mentioned above, the Mavericks installer refuses to install over Leopard Mac. But how does the installer know your drive contains Leopard, and not Snow Leopard or later? It turns out that the installer simply checks a particular file--/System/Library/CoreServices/SystemVersion.plist--on the destination disk to check the version of OS X currently installed on that disk.
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