Verify your storage device
This is a good opportunity to check the health of your storage device. Launch Disk Utility (found in Applications/Utilities), and select your startup partition in the left column (located under your storage device’s name; it may have the same name if there’s only one partition on your storage device). Then click Verify Disk.
Any problems require you to boot from a different startup volume in order to make repairs. Lion, Mountain Lion, and Mavericks user can boot into OS X Recovery (a.k.a. Lion Recovery in Lion) by holding down Command-R at startup. You can then use Disk Utility to repair your storage device. Snow Leopard users can use the installation disc as a boot drive and run Disk Utility.
To perform a more thorough examination of your Mac hardware, you can run Apple Diagnostics (for Macs released on or after June 2013) or Apple Hardware Test (for Macs released before June 2013). If you find a problem using these tests, you may need to see a Genius at your local Apple Store.
Install OS and Apple app updates
It’s a good idea to update your pre-El Capitan OS to the latest version (for example, as mention earlier, from Snow Leopard 10.6.6 to 10.6.8). You can do this either in the Updates section of the App Store or through Software Update in the Apple menu.
Check for updates to your important third-party apps
Most developers work hard to make sure their software is compatible with the new version of OS X, so often updates are released prior to the upgrade’s released date.
If you buy your software through Apple’s App Store, you can check for updates there by launching the App Store app and clicking on Updates. For apps you didn’t buy in the App Store, you’ll need to check each developer’s website on your own. You can also try checking RoaringApp’s software compatibility list.
There’s a reason I used “important” in the title for this section. If you have a lot of apps that you didn’t buy in the App Store, it can take a lot of work to find updates. So first update the apps you use most often, and then you can update the other apps when you have the time.
Back up your data
Anything you’re about to make major changes to your Mac, you need to back up your data. You can use Time Machine or a third-party backup app, or you can clone your storage device using a utility like SuperDuper or Carbon Copy Cloner.
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