MacBook Pro stuck in bootloop
MacBook Pro Model A1278 will not boot up after High Sierra update.... stuck at Apple Logo and full progress bar screen.
MacBook Pro, iOS 11.2.6
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.