Unexplored:

https://archive.org/details/macos-collection

Reinstall macOS via the internet from Apple (e.g. for a new hard drive) ∞

Alternatives:

Creating a bootable USB stick
Via the App Store: update 10.15 Catalina (other versions)

The following instructions rely on your macOS still being available from Apple. If they remove this functionality (e.g. their servers change or they stop hosting the file), then hopefully an alternative is possible.

One-time ∞

2022-11-18 on macOS 10.15 Catalina on an Apple Macbook Pro A1278 - 13" Laptop

How to reinstall macOS
Reinstall macOS -- macOS 10.15 Catalina (other versions)

Turn off
Turn on
Hold option-command-r
Select Disk Utility, then click Continue.
Select your startup disk on the left, then click Erase.
- https://support.apple.com/en-us/HT208496
Click the Format pop-up menu, choose an APFS or Mac OS Extended format, enter a name, then click Erase.
Important: Erasing the disk removes all the information from the disk. Be sure to back up the information you want to keep to an external device.
After the disk is erased, choose Disk Utility > Quit Disk Utility.
Select Reinstall macOS, click Continue, then follow the onscreen instructions.
It prompts for a WiFi password if necessary.

Creating a bootable USB stick ∞

Create a bootable installer for macOS [ 1 ]

2022-11-18 on macOS 10.15.7 Catalina on an Apple Macbook Pro A1278 - 13" Laptop

sudo softwareupdate --fetch-full-installer --full-installer-version=10.15.7

output (with errors)

While this did give me errors, it did create /Applications/Install\ macOS\ Catalina.app.

Password:
Downloading and installing 10.15.7 installer
SUPreferenceManager: Failed to set object of class: __NSCFConstantString for key: LastRecommendedMajorOSBundleIdentifier with error: Error Domain=SUPreferenceManagerErrorDomain Code=1 "(null)"
SUPreferenceManager: Failed to set object of class: __NSCFConstantString for key: LastRecommendedMajorOSBundleIdentifier with error: Error Domain=SUPreferenceManagerErrorDomain Code=1 "(null)"
Installing: 89.0%SUPreferenceManager: Failed to set object of class: __NSCFConstantString for key: LastRecommendedMajorOSBundleIdentifier with error: Error Domain=SUPreferenceManagerErrorDomain Code=1 "(null)"
Install finished successfully

I've been told these are updated scripts which don't spit out errors. I have not tried them:

(Note that Python 2.7 is no longer in newer versions of macOS. I haven't investigated getting and installing it as 2.7.16 does come with 10.15.7 Catalina.)

https://github.com/munki/macadmin-scripts/
- See Installing a newer macOS on unsupported hardware (using munki), and you can use the regular version and not some unsupported one.
- TODO - this is in testing
https://github.com/grahampugh/macadmin-scripts
- A more feature-full version.

sudo chmod u+x /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia

# Replace "YOUR_USB_STICK" with your USB stick's volume name.  You can find it on the left column of Finder.
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/YOUR_USB_STICK --downloadassets

output

Ready to start.
To continue we need to erase the volume at /Volumes/YOUR_USB_STICK.
If you wish to continue type (Y) then press return: y
Erasing disk: 0%... 10%... 20%... 30%... 100%
Copying to disk: 0%... 10%... 20%... 30%... 40%... 50%... 60%... 70%... 80%... 90%... 100%
Making disk bootable...
Copying boot files...
Finding assets to download...
Downloading assets: 0%...Failed to download package: http://swcdn.apple.com/content/downloads/37/47/012-93797-A_EAVFI5I53D/ufdgmh16d32s3fyxfgjjbzpl8ph3odn6t7/BridgeOSUpdateCustomer.pkg 10%... 20%... 30%... 40%... 50%... 60%... 70%...
Install media now available at "/Volumes/Install macOS Catalina"

Reinstall locally, using files locally (offline) ∞

Overwriting the original macOS hard drive ∞

(not tested)

I think it is the same instructions as reinstalling macOS via the internet from Apple, but instead of holding option-command-r, hold shift-option-command-r when initially booting up. This fetches the files from the local hard drive.

I think that a regular mac installation with a working hard drive will keep a hidden partition with macOS on it, just for this purpose of reinstalling the main system. I don't think the Mac has a ROM or some such with the data.

For a new/empty hard drive ∞

This is not possible. Do one of:

Get a hard drive used for another Mac
- Preferably which used to have the macOS which your present device needs. Older versions of macOS might work, and newer version of macOS have less chance to work.
- Then Reinstall locally, using files locally (offline)
Get another Mac and do one of:
- Reinstall macOS via the internet from Apple (e.g. for a new hard drive)
- Creating a bootable USB stick
OpenCore Legacy Patcher
- While you can consider installing a version of macOS which is different than the one which came with your computer, the best success might be with the one that did. Having said that, if a version of macOS is reportedly 100% operational with your hardware, then maybe that would be best.

Last updated 2022-11-23 at 15:53:19

Footnotes

was https://support.apple.com/kb/HT201372 [ ↩ ]

spiralofhope

macOS reinstallation