Reinstall macOS

01Reinstall, or erase and reinstall?

These are two different operations, and the difference is the most important thing on this page. Apple states it plainly in How to reinstall macOS: reinstalling macOS does not remove your apps or personal data.

1. Reinstall macOS

   macOS Recovery›Reinstall macOS

   Lays a fresh copy of macOS over your existing system. Your files, your apps and your settings stay where they are. It replaces the operating system and nothing else.

   Use it when: the Mac starts but misbehaves — odd crashes, features that stopped working, a system update that did not finish cleanly. This is what most people actually want, and it is far less drastic than they expect.

2. Erase, then reinstall — a clean install

   macOS Recovery›Disk Utility›Erase → macOS Recovery›Reinstall macOS

   The internal disk is wiped first. Everything on it is gone, and there is no undo.

   Use it when: you are selling or trading in the Mac, the drive is corrupted, you are handing a machine to someone else, or a plain reinstall did not fix the problem.

02Before you start

• A Time Machine backup, and you have opened it to confirm it contains your files. Apple's instructions are at Back up your Mac with Time Machine. A plain reinstall will not touch your data, but a reinstall that goes wrong mid-way can leave the system unbootable, and that is when the backup stops being optional.
• At least 40 GB of free space, for the installer and its working files. A bootable USB installer also needs its own drive — see the next step.
• Your Apple Account password. You will be asked for it to unlock the disk or to sign in during setup, and there is no way around it from Recovery.
• A wired network connection if you have one. Reinstalling from Recovery downloads macOS over the internet, and Apple notes that some networks prevent it outright: captive portals, 802.1X and PPPoE will all block the installer. A phone hotspot usually works when a hotel or campus network does not.
• Which chip you have, because Recovery works differently on each. Apple menu›About This Mac — Chip: Apple M… means Apple silicon, Processor: Intel means Intel. More detail is in Find out which Mac you have.
• An adapter, if your Mac has only USB-C or Thunderbolt ports and your drive or network cable does not.

03Get macOS, and optionally a bootable installer

You do not need a USB drive to reinstall macOS — Recovery downloads the system for you. A bootable installer is worth making anyway if you are doing this for a living, on several machines, or on a Mac whose network you cannot rely on. Apple's own framing is that it is useful when other methods are unsuccessful, which is exactly the situation where you will not be able to get online to look up how to make one.

Step one: get the installer

Everything starts from Apple's How to download and install macOS page, which links to the current release and to older ones. Downloading from the Mac App Store gets you the full installer application, named Install macOS plus the release name, and it lands in your Applications folder.

Step two: make the bootable drive

Apple's full instructions are at Create a bootable installer for macOS. In short:

1. Use a 16 GB drive or larger; 32 GB to be safe. Apple's current wording is that 32 GB holds any macOS installer and 16 GB is enough for most earlier versions — so if you are buying a drive for this, buy 32 GB.
2. Erase the drive. Open Disk Utility (Applications›Utilities›Disk Utility), choose View›Show All Devices, then select the top-level drive in the sidebar — not a volume indented under it — and click Erase. Set Name to MyVolume, Format to Mac OS Extended (Journaled), Scheme to GUID Partition Map. If no Scheme menu appears, you have selected a volume rather than the device. The name matters: the command below expects it.
3. Run the command. Open Terminal (Applications›Utilities›Terminal) and enter the line matching your installer. Replace the release name with the one you actually downloaded — the quotes are required because the path contains spaces.

Check the name against what is in your Applications folder before running it. Terminal asks for confirmation, then your administrator password — nothing appears as you type it, which is normal — then copies the installer and makes the drive bootable. Ten to twenty minutes.

04Start up from macOS Recovery

Recovery is a small operating system stored separately from your install. It can reinstall macOS, repair or erase disks, and restore from a Time Machine backup. Which key you hold depends entirely on the machine.

Apple silicon

1. Shut down. If it will not shut down normally, hold the power button for up to ten seconds.
2. Press and hold the power button. Keep holding while the Mac starts.
3. Release it when Loading startup options appears. You will see the text Continue holding for startup options before that — do not release early.
4. Click Options, then Continue.

If you are asked to choose a volume to recover, pick Macintosh HD and click Next. If you are asked to choose a user, choose an administrator account and enter its password.

Intel

Press the power button and immediately hold ⌘+R until the Apple logo appears.

Other startup combinations

Full details, including what to do when the Mac will not start from Recovery at all, are in How to start up your Mac from macOS Recovery.

If you made a bootable installer, this is where it comes in: connect it and restart. On Apple silicon, hold the power button as above and pick the installer from the startup options rather than Options. Apple silicon will only offer it if the security policy permits booting from removable media, which is set in Startup Security Utility (Recovery›Utilities›Startup Security Utility). On Intel, hold ⌥ at startup and choose the installer volume.

05Erase the disk — clean install only

1. In the Recovery window, choose Disk Utility, then Continue.
2. Choose View›Show All Devices from the menu bar. Without this the sidebar shows only volumes, and you cannot select what you need to erase.
3. Select the top-level internal drive in the sidebar — the entry named after the hardware, something like APPLE SSD AP0512Z. Do not select Macintosh HD or Macintosh HD - Data; those are volumes inside a container, and erasing them leaves the container layout behind.
4. Click Erase. Set Name to Macintosh HD, Format to APFS, Scheme to GUID Partition Map. The button may read Erase Volume Group when a volume group is selected, which is expected and correct.
5. Confirm and wait. When it finishes, quit Disk Utility with ⌘+Q.

macOS recreates Macintosh HD and Macintosh HD - Data during installation, so do not create them yourself. Apple's own walkthrough is Use Disk Utility to erase your startup disk.

06Reinstall macOS

1. Back in the main Recovery window, choose Reinstall macOS — the label carries the release name — then Continue.
2. Read the licence agreement and click Agree, then Agree again on the confirmation.
3. Select the destination disk. If you are offered a choice between Macintosh HD and Macintosh HD - Data, choose Macintosh HD. This trips people up constantly: the other volume holds your data and is not a valid install target.
4. Click Install and wait. The Mac downloads macOS, verifies it, prepares it, then restarts on its own several times. The screen may stay black or show only a progress bar for what feels like far too long. This is normal.

Do not close the lid and do not let the machine sleep. A MacBook must be on the charger. If the installer asks to unlock the disk, enter the password you log in with.

Expect thirty to sixty minutes, and longer on a slow connection — the download is the bulk of it. Apple notes that the version you end up with depends on how you entered Recovery: the local Recovery installs the current version of the most recently installed macOS, while Internet Recovery with ⌥+⌘+R installs the latest release your Mac supports.

07Setup Assistant, and getting your data back

After the final restart the Mac shows the Setup Assistant: region, language, network, Apple Account, terms, and a user account. Straightforward, with one decision that matters.

Migration Assistant

Near the end, you are offered the chance to transfer data:

• From a Mac, Time Machine backup, or startup disk — the one you want if you have a Time Machine backup from this machine. Connect the drive and it will be found automatically.
• From a Windows PC — for switching platforms.
• Don’t transfer any information now — start empty.

Both choices are recoverable. If you skip it, you can run the same tool later from Applications›Utilities›Migration Assistant, which is also the easier moment — you can set the machine up first, confirm it is healthy, and then bring everything across.

If you are giving the Mac away

Quit the Setup Assistant instead of completing it, then click Shut Down. Leave the machine at this state so the new owner can enter their own information. Apple's guidance is in What to do before you sell, give away, or trade in your Mac.

08After the install

1. Update. System Settings›General›Software Update. A freshly installed Mac is behind on point releases and usually on Safari too. Install everything and reboot as asked.
2. Turn on FileVault. System Settings›Privacy & Security›FileVault. Without it, anyone who takes the laptop and pulls the drive reads everything. Store the recovery key somewhere other than this Mac.
3. Turn on the firewall. System Settings›Network›Firewall. It is off by default, and it is a single switch.
4. Recheck Time Machine. System Settings›General›Time Machine. A reinstall can orphan the existing backup set; point it at the same drive and confirm the first backup after the reinstall completes.
5. Check that two-factor authentication is on for your Apple Account. If you are signed in somewhere you no longer control, this is the moment to remove that device.
6. Reinstall what you actually use. Apple's own app comes back with the system; everything else is a decision. The list, with links that point at the vendors, is on InstallShelf.

09When something goes wrong

Apple documents these well, so the links below go straight to the source rather than paraphrasing it.

The installer cannot see the disk

Quit the installer, open Disk Utility from the Recovery window, and erase the startup disk — see the erase step above. If Disk Utility cannot see or erase the disk at all, the drive has likely failed and the Mac needs service. Use Disk Utility to erase your startup disk.

An error appears during installation

“An error occurred while preparing the installation” and its relatives are usually a network or date problem, not a broken Mac. Check the connection, confirm the date and time are correct, and try again. Apple's guide is If an error occurs while installing macOS.

The Mac will not finish starting up

Stuck on the Apple logo, a progress bar that has not moved in thirty minutes, or a circle with a line through it. Apple's diagnostics are at If your Mac doesn’t start up all the way. Before anything else, try starting up in Safe Mode — on Apple silicon, pick the startup disk from startup options and hold ⇧ while clicking Continue in Safe Mode; on Intel, hold ⇧ at startup.

The installer says the copy of macOS is damaged

This is almost always the certificate or the download rather than the file itself. Delete the installer, check the system date and time, and download it again from the App Store link on Apple's page. Do not go looking for a “fixed” copy elsewhere.

The Mac will not start at all

On Apple silicon, a Mac that will not power on or reach startup options can sometimes be revived using Apple Configurator from a second Mac connected by USB-C. This restores firmware that a failed update has left unusable. It is the last resort before a repair, and it requires another Mac, so it cannot be your only plan.