HOME
diskutil mount /Volumes/12.6
mount -uw /Volumes/12.6
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(to change base Folder to OS10.9 GenericFolder)
mv /Volumes/12.6/Users/philip/Desktop/Assets.car /Volumes/12.6/System/Library/PrivateFrameworks/IconFoundation.framework/Versions/A/Resources/Assets.car
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(to make external drive icons look like internal ones)
mv /Volumes/12.6/Users/philip/Desktop/External.icns /Volumes/12.6/System/Library/Extensions/IOStorageFamily.kext/Contents/Resources/External.icns
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(to change Big Sur volume icon / can also do this from 10.12)
mv /Volumes/12.6/Users/philip/Desktop/.VolumeIcon.icns /Volumes/12.6/.VolumeIcon.icns
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(to change System Preferences Dock badge colour ie. showing pending updates)
mv /Volumes/12.6/Users/philip/Desktop/statuslabel.png /Volumes/12.6/System/Library/CoreServices/Dock.app/Contents/Resources/statuslabel.png
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mv /Volumes/12.6/Users/philip/Desktop/Assets.car /Volumes/12.6/System/Applications/System\ Preferences.app/Contents/Resources/Assets.car
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mv Volumes/12.6/Users/philip/Desktop/libstdc++.6.0.9.dylib /Volumes/12.5/usr/lib/libstdc++.6.0.9.dylib
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then:
find /Volumes/12.6d/private/var/folders/ -name 'com.apple.dock.iconcache' -exec rm {} \;
rm -r /Volumes/12.6d/Library/Caches/com.apple.iconservices.store
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(these not necessary: if only replacing generic folder icons in Assets.car file)
kmutil install --volume-root /Volumes/12.5 --update-all --force
/Volumes/12.6/usr/sbin/kcditto
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bless --folder /Volumes/12.6/System/Library/CoreServices --bootefi --create-snapshot
diskutil list
diskutil info disk
diskutil apfs listSnapshots 47160D35-4A44-4843-930D-B6CB98E3DD3A
diskutil apfs deleteSnapshot 47160D35-4A44-4843-930D-B6CB98E3DD3A -uuid (add)
reboot

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(to change 12.6 ethernet plugin)
cd /Volumes/12.6/System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/
mv Intel82574L.kext Intel82574L.kext.bak
mv /Volumes/12.6/Users/philip/Desktop/Intel82574L.kext /Volumes/12.6/System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/Intel82574L.kext
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(to add 'JavaApplicationLauncher.framework' folder to 12.6)
cp /Volumes/10.12/System/Library/PrivateFrameworks/JavaApplicationLauncher.framework /Volumes/12.6/System/Library/PrivateFrameworks/JavaApplicationLauncher.framework