Create a macOS High Sierra bootable usb drive – without a functioning Mac

I bought an iMac back in 2011. Lately I wanted to reactivate it as a bedroom tv (I know, it's kind of sad that a device that is still capable has to be relegated to these use cases because it doesn't receive updates anymore). The problem: I replaced the hard drive with an ssd and disposed of the original leaving me with a completely blank Mac. \[Edit: Online recovery is still possible, see first comment.\] Using Linux for over ten years I was used to be able to just download an iso for any distro I like to use and flash it to a usb drive. Apple does not provide such images though which means a classic chicken and egg problem. The amount of posts a là "how to create a bootable installer on Apple Silicon" probably means I'm not the only one. It took me way to long to realize that as long as I have a Mac on which to install macOS on I also have a functioning Mac to create the install media – even if its hard drive is blank. What you need: * A Mac compatible with macOS High Sierra * Another machine with internet connection running basically any OS which is capable of burning images to a usb drive * Three different USB drives with at least 8GB of storage How to do it: 1. Obtain the macOS High Sierra Install app. This is no longer possible via Apples App Store. But you can use third party tools such as gibMacOS or dosdude1's macOS High Sierra patcher to download the file (as every .app file actually an archive). 2. Open the .app file. On macOS by right clicking and selecting "Show Package Contents". Located there should be this file: ./Contents/SharedSupport/InstallESD.dmg 3. Copy the file to a more convenient position or take not of its location 4. Flash this image onto usb drive 1. 1. On macOS you can open a terminal window, navigate to the installer app using the cd command end then enter \`sudo dd if=./Contents/SharedSupport/InstallESD.dmg of=/Volumes/{name of usb drive 1} bs=1m\` 2. On Windows (and other systems) you should be able to use Balena Etcher for instance or any other image writer. 5. (Optional at this stage) Format usb drive 2 with HFS+ (GPT partition scheme) and name it "installer\_transfer\_drive". 6. Copy the macOS installer app to usb drive 2 7. (Optional at this stage) Format usb drive 3 with HFS+ (GPT partition scheme) and name it "macOS\_High\_Sierra\_usb\_drive" 8. Make sure no other usb drive is attached to your Mac at this point (unless you can easily identify them) 9. Plug usb drive 1 into your Mac push the power button and hold down the option key on an attached keyboard 10. Select the icon for your usb drive 1. If your hard drive is blank and usb drive 1 is the only one attached, this should be the only option 11. Be patient at this part, the boot process might take some time 12. You will be presented with the usual instruction screens showing items such as "Reinstall macOS" (won't work since the image seems no longer to be available) and "Disk Utility". 13. If you haven't formatted usb drives 2 and 3 you can use "Disk Utility" to do it here. You need to transfer the installer app to usb drive 2 on another machine though. 14. Open a terminal. This is not an icon in the main menu but you can select it under "Utilities" in the menu bar. 15. Navigate to your installer app like this: \`cd /Volumes/installer\_transfer\_drive/{name of the installer .app, might depend on the localization}/\` 16. Execute this command \`Contents/Resources/createinstallmedia --volume /Volumes/macOS\_High\_Sierra\_usb\_drive\` and follow the instructions 17. When the process is finished which might take a lot of time shut down your Mac. 18. Leave only usb drive 3 attached and boot it up again while holding the option button. 19. Select the install media and install macOS High Sierra as normal.