For the majority of users, reinstallation of an entire OS isn’t something they need to deal with. Ever. That being said, there may be instances where it’s required. In those cases, you can indeed re-install OS X from scratch, and here’s the basic overview of what you need to do.
NOTE: This is critical. BACK UP everything. Read that again, and then read it once more. Be certain you have a non-Time Machine-backup of every piece of data that you will need. This includes (but is in no way limited to) documents, music, other media, application installers, and your mail, contacts, and calendars. Time Machine backups may be restore-able, but it’s highly likely that they will not be if you reinstall the OS, so use some other tool to get your data onto removable media first.
NOTE 2: Is this trip really necessary? Usually, you do not have to install a fresh copy of OS X from scratch unless you’ve seriously messed up your Mac. Starting with 10.7 (Lion), the OS installs a hidden partition on your hard drive which permits emergency reinstallation while keeping everything it can keep in place. If you’ve just corrupted part of OS X, you don’t have to completely wipe the drive and start over. Just reboot, holding CMD+R from the time you hear the “bong” sound until you see the Apple logo and spinner show up on the screen. That will boot you into the Recovery Volume, which will allow you to re-install the OS in-place (by choosing Install Mac OS X), retaining apps and settings whenever it can.
Now, if you have trashed the OS – perhaps you got infected with malware or something else took out most of the OS itself – then a fresh installation may be the only fix available. If you’re not familiar with installing OS’s (Linux, Windows, OS X, etc.) then go to an Apple Store and let the Geniuses do it for you. If you have installed OS’s in the past, then a fresh install of OS X is actually quite easy to perform.
If you already have Lion or Mountain Lion installed, you have two choices:
1 – Use the Recovery Disk by holding CMD+R from the time you hear the bong until the Apple logo and spinner appear (just like for an in-place refresh), or
2 – Get hold of (or create) a USB or DVD OS X installer.
Then, after you are sure you backed up everything from the existing install (since I know you skipped over that part earlier), you can begin.
Boot into the Recovery Disk with CMD+R or boot to the DVD or USB image by holding down Option when you hear the bong and holding it until the Apple logo and spinner show up, then selecting the USB drive listed as “EFI Boot” or the DVD drive.
Once booted into the Lion or Mountain Lion installer (from DVD, USB or the Recovery Disk), the first thing you’ll need to do for a fresh installation is to erase/wipe the existing OS X partition. From the startup page of the installer, choose Disk Utility. Then chose the partition that holds OS X (note: NOT the physical drive, just the partition beneath it that holds the OS X system). If you used FileVault, you’ll need to unlock the partition with your login password in order to do anything with it. Just right-click the partition in the left-hand column of the page, then choose Unlock and provide your password.
Erase the partition (you DID follow the instructions to back everything up, right?) using the MacOS Extended (Journaled) choice for how the drive should be formatted. This erases all data off the drive. Choosing to overwrite the data (called “wiping the drive”) is more secure, but can take upwards of 8 hours on large disks, and is totally unnecessary on SSD’s – the choice is yours though. Once the erase/wipe is complete and you have a blank system partition, you can close Disk Utility.
Then, back on the start page for the installer, choose to Install Mac OS X. This will guide you through the basic configuration information. Choose the same partition you just formatted as the install partition, give the installer a few other pieces of data, then sit back and relax. In about 10 minutes, the installer will reboot the system.
Do not hold down any keys during this reboot, and the Mac will boot into the newly-installed OS X partition, install remaining components of OS X, and walk you through the standard OS X first-start routine.
You now have a newly-installed version of OS X on your machine, with no trace of any of the old apps, settings or data. At this point you should immediately go to the Apple Menu, select Software Updates and let everything download. Do this at least twice, but optimally you should do this until no updates are available. On Mountain lion, this will take you to the App Store, follow the same idea and install any/all updates that are waiting.
Note: Unlike other OS’s, when you download OS X you get the very latest version of the software, not just the original build that was released when the OS first came out. That means that the number of updates you’ll need is going to depend on when you downloaded the OS X image you are using for the reinstall, so keep running Software Update until no other updates are available for you.
Now you can install applications and start restoring data from your backup to your fully renewed OS X installation.
Remember, it’s nearly never necessary to do a complete reinstall of the OS. If you’re not sure, don’t do it. Try some of the tricks in previous articles (reset PRAM and other system settings, clean up permissions, etc.) and/or bring the machine to an Apple Genius. However, if a clean reinstall is called for, at least for Lion and Mountain Lion, Apple does make it pretty painless.