Apple has designed OS X/macOS so that one can just upgrade to the next version, "over the top," seamlessly. With this kind of upgrade, all user data, settings and accounts remain as before, and that works for most users most times. However, there are occasions when a user needs to do what's called a "Clean Install." This is like setting up a Mac as if it first came out of the box and then personal data is restored. This article, after a brief introduction, will list the steps needed for that Clean Install.
There are some users who suspect that a Clean Install is a good way to approach a complete new version of OS X or macOS. Over time, a lot of cruft, that is, unused extensions, app support files, preferences and other files in your Library folder or System Library folder can become troublesome or even a security issue, like Java. Or just take up too much space. See, for example: "How to Manage the Secret Software That Google Chrome Installs on Your Mac."
In addition, sometimes the normal housecleaning process with apps like Spring Cleaning and App Zapper can't solve a particularly difficult issue, and the only way to get a fresh start is a Clean Install.
But First, Let Me Talk You Out of Doing This
A Clean Install, as you'll see below, can be a time intensive process. On the other hand, over the years, we've learned that OS X is pretty good about ignoring things that are no longer applicable. For example, in OS X Yosemite, unless a kernel extension is digitally signed by Apple it won't load.
Adam Christianson (@MacCast) has written "If you aren’t really having any issues or problems then just download the new OS, install over the top and go on your merry way."
We both agree that an over the top is generally the best idea. Those who still pursue the thought of a Clean Install usually reconsider when they realize that everything they own will be deleted from the Mac.
- The boot drive is erased.
- All user accounts are deleted.
- All the files in all the user accounts (spouse, children, family cat) will be deleted. Every document, movie, song and photo will be deleted.
- All network settings are deleted.
- Every application and its license files will be gone.
However, there is a way to do a Clean Install and restore all your data if you really want to. That's what this article is all about. In summary, you will:
- Make thorough backups of your entire boot drive in several ways.
- Erase your boot drive.
- Install OS X El Capitan (or macOS Sierra)
- Restore your network settings, user accounts and all data. That means email, photos, music...everything.
I suggest you read this entire article first, understanding all the steps before you begin. Steps #5 and #6 require some preparation, are particularly tricky and must be handled with care. Ready? Here we go!
The Clean Install Process
Step #1. Prepare and Verify. Disconnect any unnecessary peripherals such as printers, tablets, microphones, etc. Try to work with just attached drives and a wired mouse and keyboard. Identify which items are backed up to iCloud. System Preferences > iCloud.
Use Disk Utility to make sure your boot drive permissions are all repaired and that the disk directory is verified as good. This will make sure your Time Machine backup doesn't get scrambled. Then you're ready for the next step.
Step #2. Backups. I recommend two methods.
Time Machine. Do one last Time Machine backup. (Menu Bar > Time Machine > Backup Now.) Later, if you have a curent Time Machine backup and something goes very wrong, you can boot from the recovery partition (CMD-R) and do a complete erase (Disk Utility) and then restore from your Time Machine archive. You'll be back where you were before you started.
File by file with Carbon Copy Cloner. This is a preferred tool by many for making an exact copy of your boot drive to an external Hard Disk Drive (HDD) or Solid State Drive (SSD). That way, you can always mount that external drive, access and copy over individual files.
2.1 You may want to have a log of all your application licenses in a copyable format so that when you restore your apps, you'll have this information handy and can paste it in. I use BBEdit.
2.2 If you try to take a short cut and just copy back in your Library folder, to make licensing easier, you'll be defeating the idea of a Clean Install. That's because the personal Library folder is one place that typically needs a thorough housecleaning.
2.3 Take note of all your mission critical apps and make sure they're all compatible with El Capitan. A great resource for that is RoaringApps.com. Barring sufficient information there, you'll have to visit the developer's website.
2.4 Some simple apps can be copied back to the Applications folder in El Capitan and run just fine. Others will require an installer that places supporting files on your Mac in various places, for example, your personal Library folder.
Catalog these apps so that you'll know which ones came from the Mac App Store and can be readily recovered and those which require you to download a new installer from the developer's website.
2.5 Note that Carbon Copy Cloner takes care of accounts on your Mac other than your own. For example, your spouse. Dont forget those!
Next page: Steps #3 to #7: Download, Install and Restore.