In case Mac restarts to a setup assistant, but you intend to give it away, hit Command + Q to quit assistant without finishing the setup. Doing so might make your mac restart and show the progress bar several times, and your screen could run empty for minutes a number of- times. Give the installation time to finish and do not close the lid or press the sleep button during an ongoing installation.This will delete everything, thus the reason why it is important you first backup your data before reinstalling a mac OS High Sierra. If the installer does not find your disk or claims that it cannot install on your volume or computer, You then might have to erase your disk. If asked by the installer to unlock your disk, key in the password you use when logging in to your mac. Press “Unlock” or “Continue” if you are using FileVault. Select “Macintosh HD” or the name of your hard drive – The MacOS Sierra will be reinstalled to this drive.Hit “Continue” as you walk through all the setup screen.Click “Reinstall MacOS” on the “MacOS Utilities”.Once the boot sound is heard, concurrently hold COMMAND + R. Steps for Reinstalling MacOS High Sierra Reinstall Mac OS The most convenient approach to data backup is using a Time machine. Although reinstalling a MacOS does not have an impact on your files, things could go wrong.