Hopefully you have access to a second Mac system. What we want to try is either connect your system to the other using Target Disk Mode or from the other system create a bootable OS installer drive so we can run Disk Utility on your systems drive.
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Here’s how to connect your system to the other in [https://support.apple.com/en-us/HT201462|How to use target disk mode to move files to another computer] and here’s how to setup a USB thumb drive [https://www.macworld.com/article/3092900/macs/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive]
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Here’s how to connect your system to the other in [https://support.apple.com/en-us/HT201462|How to use target disk mode to move files to another computer] and here’s how to setup a USB thumb drive [https://www.macworld.com/article/3092900/macs/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive].
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Having a better idea on what your exact system is will help focus things on what is needed.
Hopefully you have access to a second Mac system. What we want to try is either connect your system to the other using Target Disk Mode or from the other system create a bootable OS installer drive so we can run Disk Utility on your systems drive.
Here’s how to connect your system to the other in [https://support.apple.com/en-us/HT201462|How to use target disk mode to move files to another computer] and here’s how to setup a USB thumb drive [https://www.macworld.com/article/3092900/macs/how-to-create-a-bootable-macos-sierra-installer-drive.html|How to create a bootable macOS Sierra installer drive]