(x-post from /r/chrultrabook)

Original Post

Before you start

This guide is for the i3 model of the C720

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Requirements:

• At least a 32GB SSD (64GB+ Recommended)
• USB Flash Drive (8GB Minimum)
• Some patience
• This Clover folder
• Windows PC to create install media
• OS X Install Media with Clover installed (Easiest Method)
• This folder (Tools and kexts for post install)
• OSX_10.14.x_Installer.hfs (cannot and will not provide links to this)
• External mouse for installation
• Coreboot with Tianocore BIOS

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Known issues:

• HDMI audio is broken, there is no fix.
• Backlight settings don't work or exist, I do not care to try to fix them or provide a method of fixing them, as I use full backlight all the time anyway.
• Some people claim to have spotty wifi but personally, I've never ran into that problem.
• The modified kext I provided for WiFI (IO80211Family.kext) sometimes has issues picking up 5GHz networks. I've never run into this problem, but there are people online who have. Just a heads up.
• Touchpad right clicking doesn't work - as a workaround, long press on an item. (That may be stock MBA/MBP functionality, but I've never owned a real Mac, so I don't know if it is or not)

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Part 1: Setting up the install environment and installing OS X

1. Create your OS X install disk using Clover Boot Disk Creator
2. After everything is set up on your USB drive, delete everything that's in the clover partition of the flash drive and replace it with my clover folder
3. Boot from the installer USB and select the OS X install option
4. Open disk utility once setup is loaded and erase your SSD completely. Partition it as HFS (It will automatically convert to APFS after install)
5. Let the install do it's thing.

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There may be a bug where your external mouse will constantly keep clicking wherever your mouse pointer is on the installer, to fix this, rename or move the VoodooI2C kext from the kexts folder on your clover disk (do not delete it, it is necessary to get touchpad working after install). Your mileage may vary.

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Part 2 - Post install, eternalizing Clover

1. Once everything is installed, go through the system setup. Many things will not be working (Wifi, Audio, etc)
2. Do not setup your iCloud account (yet)
3. Get to the desktop and install the tools (link above) to your /Applications/ folder and place the kexts I provided in that same zip in /Documents/Kexts
4. Using Clover Configurator, mount your EFI partition on your SSD and replace everything with my clover folder or the one from your USB drive.
5. Unmount your EFI partition (for now)

6. Using Kext Utility, install the kexts that you placed in /Documents/Kexts/

   1. If you moved VoodooI2C kext or deleted it in part 1, put it back in the clover folder. There is a copy of it in the tools + kexts zip if you deleted it.

7. Verify everything installed correctly (if it did, you'll see a build ID for the kext cache in the Kext Utility's main window)

8. Open Mac Optimizer and rebuild system caches (second pane in the window)

9. Remove your USB drive.

10. Reboot from SSD.

Part 3 - Everything works so far, now what?

1. After you have installed all the proper kexts to the proper locations, your WiFi should be working, your audio should be working, and all graphic acceleration should be working as well. Verify that this is the case.
2. Open up your Clover Configurator again, mount your EFI partition and open up your clover config.plist in that program.
3. Go to the SMBios page and click the dropdown menu on the right hand side (under model lookup) and select MacBook Air (6,1). This will shake your device identifiers and such.
4. Generate a new serial number.
5. Check the serial number at this website. It should come up as an invalid serial number. If it does not, repeat step 4 until you do get one that is invalid.
6. Replace the first 12 digits of "Board Serial Number" with the serial number you checked (that is invalid)
7. Keep this window open, but open a terminal window and type "uuidgen" minus the quotes.
8. Copy the UUID that is generated into the smUUID box in Clover Configurator.
9. Go to RT Variables and make sure that ROM is set to UseMacAddr0, MLB is blank, BooterConfig is 0x28, and CsrActiveConfig is 0x67.
10. Save your config.plist and unmount your EFI drive.

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Part 4 - Finalizing everything

1. From this point, you can connect to the internet (if you haven't already) and install any potential updates. Your kexts should carry over doing a minor upgrade (ex. 10.14 to 10.14.5). If you plan on installing Catalina Beta (I cannot verify that anything works on 10.15, YMMV)
2. After you update, you can set up your iCloud account and iMessage / FaceTime / etc.
3. If you receive a message to call apple support and use the reference number they provided during activation of iMessage / FaceTime, then do so. They will push everything through on their end and it should work flawlessly after that.
4. You should be all set up.

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Thank you for reading my guide, let me know if there are any errors or links go down, and I will edit them accordingly.

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If you have any issues, PM me and I'll do my best to resolve them. Do keep in mind, I am just a kid who wanted to install OS X on their C720 and I'm providing you my method of installation.

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Disclaimer: None of the kexts or programs included in these folders are my work. They are all free software. If any of the developers of this software want me to remove it, just send me a PM. I will do my best to satisfy your request.