MacOS Startup Progress Bar Stuck At 100%

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How to Fix a Mac That Stalls on the Gray or Black Screen at Startup

How to abort a macOS installation loop

[A solution provided by user danbeacs. I have edited where needed.]

I noticed a lot of people having trouble with this.

When you've downloaded Mac OS Sierra on your MacBook but either the file is corrupted or you've run out of disk space, it's nearly impossible to get out of the loop by restarting your computer.

When you reboot, a dialogue box saying “Mac OS cannot be installed” may pop up. Click anywhere on the box and the top menubar will appear. Open Disk Utility and select the grayed out Macintosh HD (or whatever you have named your startup disk,) then press “Mount” - this will take a few moments. If you want to hedge your bet, click on Mac OS Sierra and select “First Aid”, to try and repair any issues with it.

Once your startup drive appears as a mounted drive, go back into the top menu, click the Apple Menu and select “Startup Disk.” The Mac’s HDD should appear. Select it, and then you can click “Restart’” on the installation box.

A nightmarish problem with a fairly simple solution. I learned the hard way. I hope this helps.

Did you read through the three links for a solution or just try the last item? If you've been through everything without success, then you will need to try the following:

Install El Capitan or Later from Scratch

If possible backup your files.

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

This will install the version of OS X you had installed.

in Recovery Mode I get msg "localized string cannot be found" . Is the Erase and reinsall the only solution? Does the "Error msg" mean I can't access OS on-line?