Some users report they are unable to download the complete mac OS High Sierra installer app, and instead wind up with a small 20mb version that attempts to download the remainder of the installer during the installation itself.This prevents the user from creating a USB installer for Mac OS High Sierra.
Installation requires downloading important content. Sometimes a reboot can be sufficient to remedy this error as well.
Very rarely, a mac OS High Sierra installation fails and remains stuck on a black screen, or all white screen.
If you are currently running mac OS High Sierra with a Fusion drive of standard spinning HDD, be sure to update system software when new versions arrive to receive support for APFS when it arrives.
There are mixed reports of Macs freezing, stalling, or otherwise not responding after installing mac OS High Sierra.
This is rare, but there are some reports the installation of High Sierra fails.
If the installation fails and the Mac operating system won’t boot at all, usually getting stuck on a blank gray screen, you may need to reinstall system software on the Mac.
Try rebooting the Mac and running the High Sierra installer again, or if you have a mac OS High Sierra USB boot installer drive, run the installer from there.
If the installation is truly frozen, you may need to reinstall mac OS High Sierra via Recovery Mode, by rebooting the Mac and holding down Command R and then choosing to reinstall mac OS.
This should resolve most issues with failed installation attempts.
If mac OS High Sierra will not proceed with installation on the Mac, usually with a notification along the lines of “mac OS High Sierra cannot be installed”, it is likely because the Mac is not compatible with High Sierra.
If you see an all black or all white screen, the first thing you should do is wait, since the issue may resolve itself, even if it takes a few hours.