Hello, based on what you said, I advise you to ignore the unmounted state of Macintosh HD and simply wipe it. If you have no Macintosh HD -Data volume, you need to back up files on Macintosh HD first because you will lose them when you wipe the Macintosh HD.
Because your Macintosh HD is unmounted, you should try solutions to mount it or recover data from the unmounted disk, and then back up your data.
If you have a Macintosh HD Data volume previously, proceed to wipe Macintosh HD in macOS Recovery mode straightforwardly.
Step 1: Boot your Mac into macOS Recovery mode
1. Press the power button to start your Mac and immediately press down the Command + R keys together before your Mac chimes.
2. Release the keys until you see a spinning globe and connect the Mac to the network if it isn't connected to a known network automatically.
Steps 2: Erase Macintosh HD on your Mac computer:
1. Select Disk Utility and choose Macintosh HD in the sidebar of Disk Utility.
2. Click the Erase button in the toolbar, then enter the requested details.
3. Click the Erase button in the new popup.
4. Disk Utility window showing Erase volume group confirmation. If asked, enter your Apple ID.
5. When done, quit Disk Utility to return to the Utilities window.
Step 3: Reinstall macOS on your Mac:
6. Click the Reinstall macOS option and click Continue.
7. Select a destination to install the new copy of macOS and select the startup disk you formatted earlier.
8. Click Continue to begin the reinstallation. You will see a window with a progress bar and time remaining for installing OS.
After the macOS installation process completes, your Mac restarts automatically to a setup assistant and then asks you to create an admin account.
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