How to Reinstall macOS Without Losing Data

This guide explains how to refresh your macOS operating system to fix issues while preserving your personal files, applications, and settings.

Difficulty: Intermediate Time: 1-2 Hours Best for: Fixing persistent system errors, slow performance, or corrupted system files.

Disclaimer: Steps may vary by device and OS version. Make a backup before major changes.

Quick Answer

Back up your Mac using Time Machine or another method.
Restart your Mac and immediately hold down Command (⌘) + R to boot into macOS Recovery.
Select Reinstall macOS from the Utilities window and click Continue.
Follow the on-screen prompts. DO NOT erase your startup disk when asked about selecting a disk.
Your Mac will download and install a fresh copy of macOS over your existing one, keeping your user data intact.

If this didn’t work: Your issue might require deeper diagnostics. Proceed to the Fast Checklist and Step-by-Step Guide below.

Before You Start

✅ What You Need

A complete, verified backup (Time Machine is ideal).
Your Mac plugged into AC power.
A stable internet connection (Wi-Fi or Ethernet).
Your Apple ID and password.
At least 15-20 GB of free storage and 1-2 hours of time.

❌ What NOT to Do

Do not select “Erase” or “Disk Utility” unless you intend to wipe all data.
Do not interrupt the installation process or shut down the Mac.
Do not skip creating a backup, even if you think your data is safe.
Avoid reinstalling on a nearly full drive; clean up files first.

Fast Checklist

Check that your Time Machine backup completed successfully.
Confirm your Mac is connected to a power source.
Disable FileVault if it’s enabled (can sometimes simplify the process).
Restart your Mac to see if the problem persists.
Check your internet connection speed and stability.
Confirm you know your Apple ID password.
Free up storage space on your startup disk.
Note down any critical passwords (Wi-Fi, user account, etc.).
Close all open applications before beginning.
Run First Aid on your startup disk from Disk Utility (in Recovery).

Mind Map: Reinstall Decision Path

      ┌─────────────────────────────────────────────────────────────────────────┐
      │                  Goal: Reinstall macOS, Keep My Data                    │
      └──────────────────────────────┬──────────────────────────────────────────┘
                                     │
                    ┌────────────────▼────────────────┐
                    │      Can you boot normally?     │
                    └──────┬────────────────────┬─────┘
                           │ Yes                │ No
            ┌──────────────▼──────┐   ┌────────▼─────────────┐
            │ Use macOS Recovery  │   │  Use Internet        │
            │ (⌘+R on restart)    │   │  Recovery (Option+⌘+R)│
            └──────────┬──────────┘   └────────┬─────────────┘
                       │                        │
        ┌──────────────▼────────────┐ ┌────────▼─────────────┐
        │ Reinstall current macOS   │ │ Downloads latest     │
        │ version over existing     │ │ compatible macOS     │
        │ system. Keeps all user    │ │ over the internet.   │
        │ files in /Users.          │ │ Keeps user files.    │
        └──────────────┬────────────┘ └────────┬─────────────┘
                       │                        │
                       └────────────┬───────────┘
                                    │
                         ┌──────────▼──────────┐
                         │   Installation      │
                         │   Process Runs      │
                         └──────────┬──────────┘
                                    │
                    ┌───────────────▼────────────────┐
                    │    Outcome Scenarios            │
                    ├─────────────────────────────────┤
                    │ 1. Success → Mac restarts with  │
                    │    fresh OS, data intact.       │
                    │ 2. Failure → Boot to Recovery,  │
                    │    use Disk Utility First Aid.  │
                    │ 3. Persistent Error → May need  │
                    │    erase & restore from backup. │
                    └─────────────────────────────────┘

Step-by-Step Guide

Step 1: Create a Verified Backup

What to do: Connect an external drive and use Time Machine to back up your entire Mac. Wait for completion.

Why it helps: This is your safety net. If anything goes wrong during the reinstall, you can restore your system and data exactly as it was.

Expected outcome: A completed backup appears in Time Machine. You can verify by opening Time Machine preferences.

Step 2: Boot into macOS Recovery

What to do: Shut down your Mac. Press the power button, then immediately press and hold Command (⌘) + R until you see the Apple logo or a spinning globe.

Why it helps: Recovery Mode loads a separate mini-operating system with tools to repair and reinstall macOS.

Expected outcome: You see the macOS Utilities window with options like “Restore From Time Machine Backup,” “Reinstall macOS,” “Disk Utility,” etc.

Note: If Command+R doesn’t work, try Option+Command+R to boot into Internet Recovery, which downloads the latest macOS compatible with your Mac.

Step 3: Run Disk Utility First Aid (Optional but Recommended)

What to do: From the Utilities window, select Disk Utility. Choose your startup disk (usually named “Macintosh HD”) and click “First Aid.”

Why it helps: Checks and repairs file system errors on your disk, ensuring a stable foundation for the reinstall.

Expected outcome: A report stating “The operation was successful.” If errors are found and repaired, run it again until it reports success.

Step 4: Reinstall macOS

What to do: Quit Disk Utility, go back to the Utilities window, and select “Reinstall macOS.” Click Continue and follow the prompts.

Why it helps: This installer replaces your system files with fresh copies while preserving your user data partition.

Expected outcome: You’ll be asked to select a disk to install on. CRITICAL: Select your main startup disk (e.g., “Macintosh HD”) without erasing it. The installer will then download and install.

Step 5: Complete Setup Assistant

What to do: After installation, your Mac will restart. Follow the on-screen Setup Assistant steps. You can usually log in with your existing user account.

Why it helps: Re-establishes your user environment and settings.

Expected outcome: You arrive at your desktop. Your files, applications, and most settings should be present. Some system preferences may be reset to defaults.

Common Causes

Corrupted system files: macOS runs slowly, apps crash, or you see strange error messages.
Failed software update: An update was interrupted, leaving the OS in an unstable state.
Malware or adware infection: Persistent pop-ups, browser hijacking, or system instability.
Disk errors: Your Mac may fail to boot or you see a prohibitory symbol on startup.
Too many system modifications: Issues after installing kernel extensions or deeply modifying system files.
User directory corruption: Problems logging in or with specific user accounts.

Diagnostics

macOS Built-in Tools

Disk Utility First Aid: Run from Recovery Mode to check the file system integrity of your startup disk.
Apple Diagnostics: Restart and hold D to test hardware. This can rule out physical disk failure.
Console App: Check system logs (in /Applications/Utilities) for recurring error messages that point to system file issues.
System Information: Review storage (Apple Menu > About This Mac > Storage) to confirm you have enough free space for the reinstall.

Fixes by Symptom

Symptom: Mac won't boot past Apple logo

Likely cause: Severely corrupted system files or disk error.

Best first fix: Boot to Recovery (Cmd+R), run Disk Utility First Aid, then reinstall macOS.

Next fix: Use Internet Recovery (Option+Cmd+R) for a fresh installer, or restore from backup.

Symptom: "Application is damaged" errors

Likely cause: Gatekeeper or system security files are corrupted.

Best first fix: Reinstall macOS to replace core security components.

Next fix: Boot into Safe Mode (hold Shift), then attempt the reinstall.

Symptom: Extremely slow performance

Likely cause: System file bloat, Spotlight indexing issues, or failing drive.

Best first fix: Reinstall macOS to refresh system files.

Next fix: After reinstall, if slowness persists, check hardware (SSD health) via Disk Utility.

Symptom: Can't install updates

Likely cause: Update catalog or installer files are corrupted.

Best first fix: Reinstall the current macOS version to create a clean baseline.

Next fix: Use the startosinstall command via Terminal in Recovery for more control.

Advanced

Using `startosinstall` Command in Terminal (Recovery)

In Recovery Mode, open Terminal from the Utilities menu. Use the command startosinstall --eraseinstall --newvolumename "Macintosh HD" to perform a clean install that automatically migrates data. This is more advanced and requires specific knowledge.

Creating a Bootable Installer for Reinstall

You can create a USB installer using another Mac. This allows you to reinstall without downloading the entire OS during the process, which is faster and more reliable if internet is unstable.

Reinstalling on a Separate APFS Volume

macOS can install a new system on a separate APFS volume on the same disk. You can then boot between the old and new systems to migrate data selectively. Use Disk Utility in Recovery to add a new APFS volume.

Dealing with Firmware Passwords

If a firmware password is enabled, you cannot boot into Recovery. You must disable it first via Apple ID or at an Apple Store.

Prevention & Best Practices

Keep regular, automated Time Machine backups.
Maintain at least 20% free space on your startup disk.
Install macOS updates promptly but wait a few days after major releases to avoid early bugs.
Use antivirus or anti-malware software if you download files from the internet frequently.
Avoid modifying system files and directories unless absolutely necessary.
Use the “Software Update” preference pane instead of installing third-party system updaters.
Before any major OS upgrade, clone your drive using SuperDuper or Carbon Copy Cloner.
Enable FileVault for encryption, but ensure you know the recovery key.

FAQ

Will reinstalling macOS delete my photos, documents, or apps?

No, if you follow the steps above and do NOT erase your disk, the reinstall process preserves all data in your user folder. Some system-level apps may need to be reinstalled, but most user applications remain.

What's the difference between "Reinstall macOS" and a clean install?

“Reinstall macOS” overlays new system files on top of your existing installation, keeping user data. A clean install involves erasing the entire disk first, which deletes everything.

Do I need my Apple ID password?

Yes, the installer will ask for the Apple ID associated with this Mac to verify the software license.

How long does it take?

It depends on your internet speed and Mac model. The download can take 1-2 hours, and installation another 30 minutes.

What if the reinstall fails or gets stuck?

Restart and try booting into Internet Recovery (Option+Cmd+R) for a different installer source. If it persists, the disk may be failing.

Will it upgrade my macOS version?

Using Command+R reinstalls the version that came with your Mac or the latest you had installed. Using Option+Cmd+R installs the latest macOS compatible with your Mac.

When should I stop and ask a professional?

If you repeatedly see disk errors in First Aid, hear unusual clicking from your Mac (hard drive failure), or if the reinstall fails multiple times, it's likely a hardware issue. Seek professional Apple support.

References / Sources

Apple Support Documentation: How to reinstall macOS
Apple Support Documentation: About macOS Recovery
Apple Support Documentation: Use Disk Utility to repair a Mac disk
Apple Platform Security Guide (FileVault and security concepts)
macOS User Guide: Back up your Mac with Time Machine

Menu

How to reinstall macOS without losing data

AI Assistant

Was this tool helpful?