I was able to fix my issue without losing data by running First Aid (from Disk Utility) when the laptop was in the crashed state. I got exactly the same problem: after 8-9 minutes got the message to restart because the update didn't work. Then I restarted in recovery mode, and that finally worked.
If the restarting in Safe mode doesn't do the trick (it may...) If I were you I would use Recovery Mode and restore from a Time Machine backup. So, now I decided to wait & see and ignore the apple store message to update, until I read some good news. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
If I remember correctly you have the ability to restore from a specific point in time. Would you like to answer one of these unanswered questions instead?
My internal SSD showed up just fine, unlike what happened to this poor chap.
Apple also had me try to restart in Safe Mode, but the Apple icon got stuck at 100% progress (3 minutes waiting).
So, since my drive appears to be intact (I used Terminal to browse my data), is it possible to undo the update or reinstall mac OS High Sierra and have the system work as it was before the update (apps and data still accessible)?