Erase and reinstall macOS refers to wiping your Mac’s hard drive completely clean and installing a fresh copy of the macOS operating system. This process deletes all data, apps, and settings on your Mac and restores it to its original factory settings.
There are a few reasons why a user might want to erase their Mac and reinstall macOS:
- To fix performance problems or system errors that can’t be resolved any other way
- To remove malware or viruses that antivirus software can’t eliminate
- To wipe all personal data before selling or gifting the Mac
- To clear up storage space by deleting unnecessary files and apps
- To revert back to an original, clean installation of macOS
Reinstalling macOS gives the computer a fresh start and often improves performance and stability. However, it also deletes all user data, so it’s important to back up anything you want to keep before proceeding.
Back Up Important Data
It is crucial to back up your important files and data before erasing and reinstalling macOS. When you erase the drive, everything will be deleted, including your documents, photos, music, applications, settings and more. The only way to preserve this data is to make a backup.
Apple recommends using Time Machine to back up your Mac before reinstalling the operating system. Time Machine is the built-in backup utility in macOS that allows you to restore your entire system from a backup. Connect an external hard drive and enable Time Machine to automatically back up your files (1). If you need to restore your data after erasing, you can easily retrieve files from your Time Machine backup.
Another option is backing up important files to cloud storage services like iCloud or Dropbox. You can manually drag files to sync to the cloud before erasing your Mac. This provides an additional layer of protection for your most important data.
How to Erase and Reinstall macOS
Here are the step-by-step instructions for erasing and reinstalling macOS:
- Boot into Recovery Mode by restarting your Mac and immediately pressing and holding Command-R until you see the Apple logo or globes. This will boot you into the Recovery system.
- Once in the Recovery screen, select Disk Utility and click Continue. This will open Disk Utility.
- In Disk Utility, select your startup disk from the sidebar and click the Erase button. Give your disk a name, select Mac OS Extended (Journaled) as the format, and click Erase to wipe your startup disk.
- After erasing, quit Disk Utility to return to the Recovery window. Select Reinstall macOS and click Continue to start the installation process.
- Follow the onscreen prompts to install the latest macOS your Mac supports. The installation will take some time to complete.
- Once the installation is finished, you will be walked through setting up your Mac from scratch, including creating a new user account.
This process will completely erase your startup disk and do a clean install of macOS from Recovery Mode. Make sure to back up your data before erasing the disk. Reinstalling macOS can help resolve problems and improve performance on older Macs.
What Gets Deleted
When erasing and reinstalling macOS, all user accounts, apps, files, and system settings are deleted. This includes everything in the user’s home folder like documents, photos, music, downloads, desktop items, and any custom settings. Any apps downloaded from the App Store or Mac App Store will need to be redownloaded and reinstalled. The erase process also deletes all system preferences, Dock customizations, network settings, and other modifications made to the default macOS settings. Essentially a clean slate is created, removing all traces of user data and activity (Apple Support). The erase function overwrites all data sectors to prevent any chance of recovery, effectively wiping the hard drive or SSD completely clean.
Some key items erased during the reinstall process include:
- User accounts and home folders
- Downloaded applications
- Documents, music, photos, videos
- Browser history, bookmarks, caches
- Email accounts and messages
- System settings and preferences
- Network configurations and passwords
So everything that makes the Mac personal to the user is removed. It is a complete reset to factory default settings.
What Stays Behind
Even after erasing the drive and reinstalling macOS, some data may persist on the Mac. This includes:
- Some system logs and caches stored in the /Library folder. These contain technical data about the system and applications.
- The EFI partition that contains firmware settings is not deleted.
- Recovery Partition that enables macOS recovery is also not deleted.
- NVRAM contents such as network settings and PRAM settings are retained.
- User accounts and some preferences are stored in Open Directory and will remain unless Open Directory is also erased.
So in summary, although most data is deleted when erasing the drive, some supporting system files and partitions will stay behind. These contain settings and caches needed for the system to operate properly.
Reinstall macOS Versus Upgrade
There are some key differences between completely erasing and reinstalling macOS versus upgrading to a newer version of the operating system. Upgrading installs the latest macOS release but leaves all of your files, settings, and applications in place. Reinstalling starts fresh by erasing your entire drive and installing a clean copy of macOS.
Upgrading is quicker and easier, allowing you to get the latest features and fixes without having to migrate data or reconfigure your settings. However, upgrading can potentially carry over problematic files, cache issues, and other errors that a clean install would remedy. Upgrades also tend to use more storage space over time.
Reinstalling macOS provides a blank slate, which can resolve performance problems, system errors, and storage clutter that occur after prolonged usage. It also removes any malware or unnecessary hidden files left behind. The downside is that you’ll need to reinstall apps, recreate settings, and move your personal files back after reinstalling the OS. You’ll also lose any customizations made to the previous OS installation.
In general, upgrading is preferable for quick version updates or if you want to maintain your current setup. Reinstalling makes more sense if you’re troubleshooting severe system issues or preparing to sell your Mac. Users who are meticulous about optimizing performance often do a clean install after 1-2 major macOS upgrades.
After Reinstalling macOS
Once the process to erase and reinstall macOS is complete, there are several steps users should take. First, they will need to go through the setup process again, which includes connecting to a wifi network and creating a new user account. Since erasing the disk deletes all user accounts, users will have to set up a new account from scratch.
Next, users will want to reinstall all their necessary apps from the App Store or other sources. Because the entire system has been wiped, no apps will remain installed. Users should make a list of the key apps they rely on for work and personal use before erasing the disk so they know which ones to redownload.
Finally, users will need to restore their files and data from a backup. Before erasing, users should make sure they have a complete backup of their documents, photos, music, and other files either using Time Machine or another backup service. Once the fresh OS is installed, users can connect the backup drive and restore the files to their new user account and folders.
According to Apple’s support article on reinstalling macOS (https://support.apple.com/guide/mac-help/erase-and-reinstall-macos-mh27903/mac), setting up the new account and restoring files from a backup are key steps after the OS reinstallation is complete.
Potential Issues
There are a few potential issues that may occur when erasing and reinstalling macOS:
Installation failure – You may see an error like “An error occurred while preparing the installation” or “This copy of the Install OS X application can’t be verified.” This is often caused by a damaged or incomplete installer file. Try re-downloading the installer from the App Store or Apple’s website (https://support.apple.com/en-us/102531).
Stuck on boot screen – If your Mac gets stuck on the Apple logo or spinning globe screen during installation, try restarting and immediately holding Command + R to boot to recovery mode and reinstall macOS from there.
Lost data – Be sure to properly back up your data first, as erasing the drive will delete everything. If you forgot to back something up, data recovery software may be able to help retrieve it.
Installation hang or freeze – If the installation seems to freeze for a long time, give it some time as it may be still working in the background. But if it’s frozen for 30+ minutes, you can force restart your Mac and try the install again.
Incompatible software – After reinstalling macOS, some older software may no longer be compatible. Check for and install software updates as needed.
If you run into an error or issue during reinstallation, stay calm and don’t force restart your Mac unless absolutely necessary. Search for the specific error message to find troubleshooting tips. Apple’s support guides are also a great resource for resolving macOS installation problems (https://support.apple.com/en-us/HT204904).
When to Erase and Reinstall
There are several situations when it can be beneficial to erase your Mac and reinstall macOS:
- To speed up an old Mac – Over time, accumulated files and outdated software can slow down your Mac’s performance. Wiping the hard drive and reinstalling a fresh macOS can help improve speeds on an older machine.
- To fix persistent issues – If you have tried other troubleshooting steps but are still experiencing crashes, bugs or performance problems, erasing and reinstalling macOS often resolves stubborn issues.
- Before selling a Mac – You’ll want to securely wipe all your personal data off the Mac before selling it. Erasing and reinstalling macOS is the best way to fully reset it to factory settings.
- To change file systems – If you want to switch from HFS+ to APFS, the recommended method is to erase your drive first and then install macOS, which will convert the drive to APFS.
- Major macOS upgrades – Doing an erase and fresh install when upgrading between major macOS versions, like Catalina to Big Sur, can help avoid compatibility issues.
In general, erasing and reinstalling macOS is most useful when you want to start fresh and address persistent problems, or when preparing a Mac for sale or transfer to a new owner.
Conclusion
In summary, erasing and reinstalling macOS completely wipes your hard drive and reinstalls a fresh copy of the operating system. This can be useful for troubleshooting performance issues, fixing persistent bugs, removing malware or bloatware, or preparing to sell your Mac. However, the process also deletes all your files, settings, and applications, so it’s critical to fully back up your system first.
While reinstalling macOS provides a blank slate and fresh start, restoring files and settings afterwards can be tedious. Upgrading to a new version of macOS is often easier. Consider erasing only if you’re experiencing significant systemic problems. And remember – always have a current, bootable backup before proceeding with any major operating system changes.