This page covers the full process of how to update and install the latest macOS Big Sur, Catalina, Mojave, etc. on Mac without losing any data. If you need a detailed guide, follow through this page to get the desired macOS installed on your computer now:

All personal data and applications that you have manually applied to the hard disk will be erased. To protect your personal data, create a backup with Time Machine or similar software. Finally, when you are ready to reinstall the operating system, click the Apple logo in the menu bar at top of the screen and select Restart or Shut Down. When your Mac starts reinstalling MacOS Catalina, it’s going to take an hour or two to finish, so stay patient. The good news is that your files and apps — unless corrupted — should be saved. Reinstalling macOS without losing files is very easy. So, if you have been asking how to reinstall macOS without losing data, the steps highlighted above is the right way to do it. If you lose data in the reinstallation process, you can easily recover them using iBeesoft Data Recovery for Mac. Related Articles.

Workable Solutions	Step-by-step Troubleshooting
Phrase 1. Preparations	2GB+ memory and 9GB+ storage space left on Mac > 45% + battery > Connect to internet...Full steps
Phrase 2. Change a USB port	Start Mac from macOS Recovery > Select 'Reinstall macOS' > 'Continue'...Full steps
Phrase 3. Recover Lost Data	After macOS installation, run EaseUS Mac data recovery software > Scan and recover lost data...Full steps

Applies To: macOS Big Sur, macOS Catalina, macOS Mojave, macOS High Sierra, macOS Sierra, or earlier Mac operating systems

The need to update or reinstall your macOS always follow the release of a new operating system, the slow performance of your Mac, or a system crash. When it comes to reinstalling macOS, users biggest concern is losing data on their computer.

The good news is that, if you follow the instructions strictly to update your Mac Operating System, the chance to lose data on your Mac is quite slim, since a reinstallation just needs to create a new copy of the OS, your existing files stored on your Mac won't be lost. Thus, as long as you don't choose to erase your startup disk during the reinstallation, you won't lose files. However, you know what 'Slim' means - it's not 100% safe and is still at the risk of losing some data.

Then how to reinstall macOS without any data loss? To provide a double guarantee to the precious data on your Mac, you can make a copy of the files on your Mac before you perform the reinstallation, or equip your Mac with a data recovery tool that can rescue your data after the reinstallation.

Preparations you Need to Make Before You Reinstall macOS

To reinstall macOS on your computer smoothly. Follow the tips below to prepare your Mac ready for the new macOS update now:

• Leave at least 2GB memory and 8.8 GB storage space on your Mac
• Leave at least 45% battery on your Mac
• Connect Mac to the internet or WiFi
• Update your Mac Apps to the latest versions
• Backup all your personal data and even create backups for your present Mac data through Time Machine

Remember to back up your Mac to an external hard drive for preventing system update failure or unexpected troubles from happening. Then follow the detailed guide below to reinstall Mac OS via macOS Recovery mode, which enables you to reinstall macOS, restore your files from Time Machine, repair or erase a disk, and more.

How to Update & Reinstall macOS Without Losing Data

*Print this page or open this page on another device so you can refer to it later.

Step 1. Start up your Mac from macOS Recovery

• Turn on your Mac and immediately press and hold 'Command + R', 'Option + Command + R', or 'Shift + Option + Command + R'.
• Release the keys when the Apple logo, a spinning globe, or a pop-up for a firmware password appears.

Step 2. Select 'Reinstall macOS' from the Utilities Window and click 'Continue'.

Step 3. Follow the onscreen instructions to select the hard drive you want to install the OS on and start the installation.

Step 4. Don't put your Mac to sleep mode or close its lid during the installation. It's normal to see your Mac restart several times or the screen is white while you reinstall the Mac OS. Just wait patiently for the process to finish.

If data loss occurred after you reinstalled macOS, you can follow the next part to recover your missing files.

Recover Lost Data After macOS Reinstallation

If you have backed up your Mac before the reinstallation, you can retrieve your data from Time Machine, an external hard drive, or somewhere else, depending on the way you used to make the backup. If you haven't created any backups for your Mac, then you'll need the professional Mac data recovery software tool - EaseUS Data Recovery Wizard, to perform data recovery. With the help of this tool, you can easily recover the data lost due to macOS reinstallation:

Step 1. Select a location and start scanning

Launch EaseUS Data Recovery Wizard, hover on the location where you lost data, then click 'Scan'.

Step 2. Preview and select the files

Wait until the scanning completes. You can use Filter or search bar to find lost files.

Step 3. Recover lost data

Then choose a location to save the recovered data, and click 'OK'. You should store the data in another place instead of the original one.

There are lots of data recovery tools available. But EaseUS Data Recovery Wizard, one of the best Mac data recovery tools, is our pick. It fully supports all macOS systems including the newest Big Sur, Catalina, Mojave, High Sierra, Sierra, EI Capitan, Yosemite etc. It allows you to restore all lost data with simple clicks. Office files, music, videos, emails or other archives can all be restored on your own.

After months of waiting, macOS Big Sur was released in November 2020. No matter how great macOS Big Sur is, you can experience performance issues as you update your Mac. The good thing is you can downgrade to the previous macOS version.

There are two ways to revert from Big Sur to Catalina. The first one involves restoring a Time Machine backup. The second, which is a bit lengthy, explains how to downgrade to Catalina using a bootable installer. Whichever way you choose, we'll provide step-by-step instructions to help you downgrade.

Open this article on your phone to make sure you do everything step-by-step.

How to downgrade from macOS Big Sur using Time Machine

If you’ve backed up your Mac with Time Machine before upgrading to Big Sur, downgrading to Catalina will be easy. Here’s how to downgrade from macOS Big Sur to Catalina using Time Machine.

1. Back up your data

First, back up everything. Your Mac probably contains your personal data and documents that you don’t want to lose. So, to keep those alive after the downgrade, you need to back up your data.

You can use Google Drive, iCloud Drive, or any other cloud you prefer. After the backup is done, you can move on to the next step.

Before you begin reverting your Mac to macOS Catalina, it’s worth trying fixing performance issues on macOS Big Sur. Maybe you won’t need to downgrade at all.

CleanMyMac X can help identify and solve performance problems on your Mac. It’s a dedicated Mac cleaner that clears old junk and runs optimization tasks. It’s notarized by Apple, which means it’s safe for your Mac. Get CleanMyMac X for free to run a quick performance scan.

2. Erase your Mac’s hard drive

The first step is plugging your Mac into power (the downgrade process may take a while, and you don’t want your Mac to power off unexpectedly).

You’ll need to erase your Mac’s drive. This will remove everything from your Mac, but you can restore your data later from a Time Machine backup.

1. Restart your Mac (Apple menu > Restart).
2. Hold Command-R when your Mac’s rebooting. The Utilities menu should appear.
3. Choose Disk Utility.
4. Click Continue and select Startup Disk (usually located at the top of the list ).
5. Press Erase.
6. Select the APFS file format.
7. Choose GUID Partition Map and confirm.

Wait for the process to complete. Only after every piece of data is removed, can you clean install macOS Catalina.

3. Use Time Machine to restore your backup

You can now restore all your files and data and bring your Mac back to the condition it was in before you installed macOS Big Sur.

If your Time Machine backup is stored on the external drive, plug it in your Mac.

So, to restore your Time Machine Catalina backup:

• Restart your computer and hold Command-R when it reboots.
• Select the Restore From Time Machine Backup option on the Utilities window.
• Press Continue.
• Choose your Time Machine backup disk.
• Select the backup you want to restore from. Choose the most recent backup that occurred before you installed macOS Big Sur.
• Then, select a destination disk, where the contents of your backup will be stored.
• Click Restore and press Continue.

The process may take some time. Your Mac will restart running macOS Catalina.

How to downgrade from macOS Big Sur using a bootable installer

If you didn’t back up your Mac with the Time Machine, it’s too bad. But, you can still downgrade to the previous OS version. Just follow the instructions.

1. Back up your files and data

Backing up your data is important. It prevents you from losing all the files and data that have existed on your Mac's drive for a while. So, don’t forget to back up your Mac before you start downgrading to Catalina.

2. Create a bootable installer

A bootable installer will help you safely roll back to Catalina. Here’s how to create a bootable installer:

1. You can go to the App Store and search for Catalina, then download it. Quit the installer, if it tries to install the OS.
2. If your Mac already runs macOS Big Sur, download the macOS Catalina here.
3. Get an external hard drive that has at least 12 GB of available storage and plug it in your Mac.
4. Launch the Disk Utility app and erase your hard drive selecting Mac OS Extended format.

Now, it’s time to transfer your Catalina installer to your hard drive:

1. Open Terminal (Applications > Utilities).
2. Paste this command and press Enter:

sudo /Applications/Install macOS Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

The downloaded file, which is your bootable Catalina installer, should be in your Applications folder. MyVolume is the name of the hard drive; if your hard drive or flash drive has a different name, replace MyVolume with the name of your hard drive.

1. If prompted, enter your administrator password and press Enter again. Terminal won’t show anything when you type your password.
2. Follow the instructions that appear in the Terminal.
3. When Terminal says “done”, your hard drive should have the same name as the installer you downloaded (for example, Install macOS Catalina).

Quit Terminal and eject the hard drive.

Install macOS Catalina

Now, you can roll back from macOS Big Sur using the bootable installer.

• Plug your hard drive (which is now your bootable installer) into your Mac.
• Open System Preferences > Startup Disk. Choose your bootable installer as a startup disk and press Restart.

• Your Mac should start up to macOS Recovery.
• Make sure your Mac has an internet connection to download firmware updates (you can use the Wi-Fi menu in the menu bar).
• In the Utilities window, select Install macOS.
• Click Continue and follow the on-screen instructions.

Your Mac will then start installing macOS Catalina and will restart when it’s done.

Mac runs slowly after the update?

Reinstall Macos Catalina Without Losing Data Connection

If your Mac is slow after the update, but you don’t want to perform this lengthy process and revert to the previous OS, you could try a quick solution. CleanMyMac X has a helpful Maintenance feature that could fix all possible problems on your Mac.

How To Reinstall Macos Without Losing Data

1. Open CleanMyMac X.
2. Go to the Maintenance module.
3. Press View All 9 Tasks.
4. Check the boxes next to the maintenance tasks you want to run.
5. Press Run.

Wait till CleanMyMac X runs the set of fixes to speed up your Mac.

Reinstall Macos Catalina Without Losing Data Settings

Reverting to the previous macOS may seem like a terrifying task. But if you open this article on your phone or another device and follow the instructions, it will be much easier and faster. Don’t forget to clear your Mac before the backup – you will save a lot of free space and remove old clutter that slows down your machine.