I acquired an [aluminium Intel iMac](https://en.wikipedia.org/wiki/IMac_(Intel-based)#Aluminum_(2007%E2%80%932009). I proceeded to ruin the installation. Recovery still worked (if it doesn't, try some other instructions). Attempting to reinstall from recovery wants to download from the app store, but apparently El Capitan isn't in there anymore. How can I recover it?

  1. Download El Capitan from Apple. Its sha256sum is bca6d2b699fc03e7876be9c9185d45bf4574517033548a47cb0d0938c5732d59, md5 is 5040a59d785d7da609755244c4ed136c.
  2. Most guides - including those from Apple - say that you have a .app file. This downloaded file clearly isn't. Boot into recovery, then transfer the .dmg file to the Mac (try using an exfat partition for a USB drive).
  3. Use disk manager to mount the image. The partition on the USB disk didn't show up for me, so open a terminal, then:
    1. diskutil list to identify the partition
    2. cd /Volumes
    3. mkdir usb
    4. mount -t exfat /dev/disk2s1 usb
    5. Close the terminal, open Disk Utility
    6. Go to File, Open Disk Image
    7. Find the mounted USB disk, and open InstallMacOSX.dmg
  4. Create a volume to store the files if necessary. (Don't format it as FAT - it can't store files big enough!)
  5. $ cd (the volume to put files in)
$ pkgutil --expand InstallMacOSX.pkg elcapitan
$ ls -F elcapitan
Distribution*       InstallMacOSX.pkg/ Resources/
$ cd elcapitan/InstallMacOSX.pkg/
$ tar -xvf Payload
x .
x ./Install OS X El Capitan.app
x ./Install OS X El Capitan.app/Contents
…
$ mv InstallESD.dmg "Install OS X El Capitan.app/Contents/SharedSupport"
  6. Erase the disk and mount it
    # diskutil eraseDisk HFS+ elcapitan disk2
# diskutil mount /dev/disk2s2

  7. # "Install OS X El Capitan.app/Contents/Resources/createinstallmedia" --volume /Volumes/elcapitan --applicationpath "Install OS X El Capitan.app"
Ready to start.
To continue we need to erase the disk at /Volumes/MyBlankUSBDrive.
If you wish to continue type (Y) then press return:
  8. Boot to the USB drive (hold Command/Alt). If you try to install the obvious way, the installation will fail, saying the installer is corrupt. Open a terminal instead, and run:
    installer -pkg  /Volumes/Mac\ OS\ X\ Install\ DVD/Packages/OSInstall.mpkg -target /Volumes/"XXX"

That was way more complicated than it had to be! Why can't I download a bootable ISO? I'm keeping an image of the install USB stick.

= Resources =

  • https://chriswarrick.com/blog/2020/06/03/reinstalling-macos-what-to-try-when-all-else-fails/#toc-entry-4
  • https://gist.github.com/coolaj86/22d08c4b582779485e0df11c5d84063a
  • https://apple.stackexchange.com/a/232016