Make offline install media

on macOS.
Make an offline install media: You need a macOS for this. You need to download Install_macOS_12.6-21G115.dmg first, which is 12GB(12,408,060,752) bytes. It takes a long time. It says 5 hours. That is why Online install takes more than 5 hours. Download is the part, which takes time.
  1. Preparation; you need
  2. Download Install_macOS_12.6-21G115.dmg (the name depends on your target OS.)
    3. Do this on your macOS in a terminal. 
    	  $ mkdir -p ~/macOS-installer
	  $ cd ~/macOS-installer
	  $ curl
	  https://raw.githubusercontent.com/munki/macadmin-scripts/main/installinstallmacos.py
	  > installinstallmacos.py
	  $ sudo python installinstallmacos.py
    Choose the number associated your designated OS version.

    Note python should be python3 in some case. Note you need to run below command; pip might be pip3 in some case. 

    	  $ pip3 xattr
  3. (on Linux) Prepare your media
    4. It may destroy your data on the USB media. Be careful and make a back!
    1. make your media GPT from MBR(Master Boot Recorder), if it is not GPT yet. 
      	  $ sudo gdisk /dev/sdb   <-- sdb depends.
      just type w, and then Y.

      just type w, and then Y.

      Now you confirm it is gpt.
    2. (on Linux) make a proper partition table 
      	 $ gparted
      make two partitions;
      • FAT 700MB. Label: EFI. Flag: Boot, EPS.
      • HFS+ 15300MB. Label: INSTALL1206.
        • Note: FAT32 is also fine, because it will be re-formated by installer.

      Now you confirm it is gpt.
  4. (Back on macOS) create an install media.
    1. open Install_macOS_12.6-21G115.dmg with mounter.
      2. It will be mounted as /Volume/Install_macOS_12.6-21G115/
    2. Run the commnad 
      	$ sudo /Volumes/Install_macOS_Monterey/Install_macOS_Monterey.app/Contents/Resources/createinstallmedia --volume /Volumes/INSTALL1206

      createinstallmedia command.
  5. Copy all OC files to /EFI on EFI partition on the media.
    6. The necessary files depends on your system. See Dartania's OpenCore guide.
Now you have an offline install media.
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