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I am trying to rescue my dad's 2008 iMac. It is running Snow Leopard but the most recent OS it supports is El Capitan. It won't boot past a spinning progress bar on a grey screen, and will not boot into Recovery Mode or Safe Mode.

It seems to me next step is to try a bootable USB stick. I am running into walls trying to make the stick from my own computer, which is an M1 MacBook Air running Big Sur.

An El Capitan .DMG is downloadable directly from the Apple Support page, which provides instructions on how to make a bootable volume, with a major caveat On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg

I used a utility to extract the Install OS X El Capitan.app file from the .dmg and then ran the terminal command from that support page:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyUSBVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app

But this yields /Applications/Install OS X El Capitan.app does not appear to be a valid OS installer application. When I google this error some results indicate it is due to an expired file, but changing my system date to 2016 does not fix it. I assume the real problem is a system incompatibility.

Surely there is just an ISO I can get and write without all these hoops?s

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    The first thing I might try is launching the createinstallmedia with Rosetta, so it should be as if you were using an Intel Mac. The simplest way is probably to select the Terminal app in Finder, open the Get Info pane, and check "Open with Rosetta". Relaunch Terminal, and try running createinstallmedia again. Make sure to reverse the Open With Rosetta check when you're done.
    – Wowfunhappy
    Apr 4, 2021 at 1:23
  • @Wowfunhappy A very good idea, but unfortunately it yields the same error when Terminal is running in Rosetta.
    – Nelluk
    Apr 4, 2021 at 1:32
  • Does this answer your question? How do I create El Capitan installer on a Catalina (or post-El Capitan) installed Mac for use on USB boot installer Specifically, see this answer.
    – David Anderson
    Jul 10, 2021 at 17:30
  • Do you still have the Snow Leopard installation disks? The easiest might just be reinstalling that, and then let the Software update procedure do its job. Note that some versions of OS X still cost money.
    – Thorbjørn Ravn Andersen
    Jul 10, 2021 at 23:29

3 Answers 3

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Start by downloading the El Capitan dmg from this link : https://support.apple.com/en-us/HT211683

Then open the dmg and copy InstallMacOSX.pkg on the Desktop.

Then, from the terminal (Applications/Utilities) un-compact the InstallMacOSX.pkg file in a directory (Installer for example) which will be created by the following pkgutil command:

pkgutil --expand ~/Desktop/InstallMacOSX.pkg ~/Desktop/Installer

Then position yourself in the InstallMacOSX.pkg "package" created by the pkgutil command

cd ~/Desktop/Installer/InstallMacOSX.pkg

Then un-compact the structure using the tar command:

tar -xvf Payload

Finally move the InstallESD.dmg file created by the tar command above to the Desktop

mv InstallESD.dmg ~/Desktop

You must then format a GUID partition scheme USB key of sufficient size (8 GB for El Capitan) named KEY in the example and execute the following instructions :

hdiutil attach ~/Desktop/InstallESD.dmg -noverify -nobrowse -mountpoint /Volumes/install_app
hdiutil convert /Volumes/install_app/BaseSystem.dmg -format UDSP -o /tmp/Installer
hdiutil resize -size 8g /tmp/Installer.sparseimage
hdiutil attach /tmp/Installer.sparseimage -noverify -nobrowse -mountpoint /Volumes/install_build
rm -r /Volumes/install_build/System/Installation/Packages
cp -av /Volumes/install_app/Packages /Volumes/install_build/System/Installation/
cp -av /Volumes/install_app/BaseSystem.chunklist /Volumes/install_build
cp -av /Volumes/install_app/BaseSystem.dmg /Volumes/install_build
hdiutil detach /Volumes/install_app
hdiutil detach /Volumes/install_build
hdiutil resize -size `hdiutil resize -limits /tmp/Installer.sparseimage | tail -n 1 | awk '{print $ 1}' `b /tmp/Installer.sparseimage
hdiutil convert /tmp/Installer.sparseimage -format UDZO -o /tmp/Installer
mv /tmp/Installer.dmg ~/Desktop

Here you have to plug the USB key named KEY, then :

sudo asr restore --source ~/Desktop/Installer.dmg --target /Volumes/KEY --noprompt --noverify --erase

Test the key and if ok, delete the working directories and files of this operation from the Desktop.

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  • Thank you for this detailed and helpful info
    – Alexander Volok
    Jun 28, 2022 at 20:45
  • Genius, thanks a lot!!!!
    – Daniel F Jaramillo
    Jan 5 at 2:24
  • This was just amazing, thank you so much!
    – User123
    Sep 5 at 0:27
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I tried and failed to create a Catalina bootable USB on an M1 Mac using creatinstallmedia. Thread about it here:

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  • Welcome to Ask Different. You’ve posted this as an answer, which is only for things that directly answer the question asked.
    – bmike ♦
    May 7, 2021 at 1:37
  • @mikebhm I've a solution to create a Catalina bootable USB on an M1 Mac. If you create un new post with your request I'l give you the instructions.
    – user415185
    May 11, 2021 at 10:30
  • @bmike Apologies. I see my mistake, apologies.
    – mikebhm
    May 12, 2021 at 11:19
  • @Jean_JD Thanks for the offer. I don't need to do this for myself (I was trying to help others). Your solution above looks like it is applicable to other OS versions.
    – mikebhm
    May 12, 2021 at 11:23
  • Apology accepted and not required - this place takes some time to learn - there's no harm in having a few wrong answers / deleted things
    – bmike ♦
    May 12, 2021 at 11:23
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These instruction will require you to use the M1 Mac to do the downloading. Use an USB flash drive or file sharing to transfer downloads to the 2008 iMac.

Here are the steps to follow.

  1. Get Snow Leopard 10.6 installed on the 2008 iMac. Use the M1 Mac to create USB flash drive bootable Snow Leopard 10.6 installer. See this question.
  2. Upgrade the 2008 iMac to OS X 10.6.8.
  3. Use Snow Leopard to create the Install OS X El Capitan application.
  4. Use Snow Leopard to create the El Capitan USB installer (El Capitan rescue USB).

Note: In your case, you probably could avoid creating the El Capitan USB installer the and just run the Install OS X El Capitan application on the 2008 iMac.

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