You certainly do need a backup. You'll save yourself a lot of time and grief by following these tips:
1. Check the tech specs
Check out the tech specs for Mountain Lion to make sure you can run it on your current system. If you are upgrading from Snow Leopard, you should also check out will my old apps work with Lion?.
2. Run Software Update
Make sure you have the latest version of everything by running
> Software Update
before you upgrade. This will help to cut down any compatibility problems when installing Mountain Lion.
3. Backup your current install
Without a doubt the biggest cause of trouble and frustration with every OS release is among users who didn’t backup their systems before trying to install the new one. Like Lion, Mountain Lion will be delivered as a download from the App Store, and while this method is certainly convenient, corrupted downloads are not uncommon. Moreover, even when the install proceeds as expected, you want to be able to get back to your previous system if you find things are not to your liking. An OS installation is like performing major surgery on your computer and the chances of complications are always a significant risk. As the old boy scout saying goes ‘Be Prepared’. Backup, backup, backup.
4. Remove everything from your Login Items.
For Lion users, that’s
> System Preferences…> Users & Groups | Login Items
If you’re upgrading from Snow Leopard 10.6.8, go to
> System Preferences…> Accounts | Login Items
There’s a couple of reasons for doing this, but mainly it’s to ensure that you can log in without difficulty. Login Items are one of the main causes of OS problems at the best of times, but with a major upgrade like this, you don’t want any incompatible software choking the system from booting up. See Step 8 below about re-activating your Login Items after the installation.
5. Be prepared for slow download times
Apple’s servers will be hotter than a forest fire on release day, so you can expect download times to be pretty poor. At the very least, connect your computer to your router via ethernet cable rather than wifi – that’ll at least help push things along a bit faster at your end. If you can do it from the office or somewhere else that has a lot more bandwidth than the usual home connection, do so.
6. Save the installer
After the download completes the installer will run automatically. Do NOT click ‘Install’, but instead quit it. Then go to your Applications folder and copy the ‘Install OS X Mountain Lion .app’ and save it to your Downloads folder or somewhere else. Then you can run the installer in your Applications folder.
The reason for doing this is that when you install Mountain Lion the Install app in your Applications folder will self-delete. If the install shouldn’t work for some reason the first time round, you won’t have to go through the whole tiresome wait of a download from the App store again if you save a copy to your Downloads folder.
7. Update 3rd-party apps
Assuming Mountain Lion boots up just fine, the first thing you’re going to want to do (after having a nose around, of course!) is update your 3rd party apps. Start off by opening the App store to see if any that you’ve sourced from there have updates waiting for you. After that, start up all the main apps that you normally use and use their ‘Check for Updates’ option (usually in the Apps main menu), or contact the developers through their websites.
8. Test your Login Items apps
In particular, you’re going to want to test out the apps that you want to put back in your Login Items, before you put them back in there. Run each app and play around with the system for a couple of hours before deciding to put it back in your Login Items list. If you notice any weird behaviour with Finder, the Dock or Desktop backgrounds, problems with wifi or graphics, chances are one or more of your 3rd party apps needs updating to work with Mountain Lion. If there is no update yet, stop using that app till one becomes available or find an alternative. In general, it’s always best to keep Login Items down to a bare minimum. Running apps at login is a sure fire way to slow down boot up times.