Long reply, but I think it might solve the problem. I posted it on the Apple Support Forums at:
https://discussions.apple.com/message/33347385#message33347385
Well folks, I struggled through multiple more reboots that, despite me setting App Store to Hide Update, repeatedly crashed the system to the black screen and mouse cursor.
I finally found a solution that worked for me, after using Console in /Applications/Utilities and searching /private/var/install.log for 2018-001 and then lines before and after this and noticing several like this:
2018-04-27 01:01:33+08 iMac Installer[899]: recoveryPartitionVersionForVolume: could not get recovery version information for mount point /Volumes/Time Machine Backups: Unable to find boot helper partition. (-69737)
Hopefully it will work for some of you as well.
Absolutely no thanks at all to Apple.
1. Following longstanding advice from various parties, I disconnected all connected drives. In my case these were Time Machine backup drives connected by USB and by Ethernet, which I ejected and removed from Time Machine backups respectively.
2. I shut down, then after a few seconds pressed power button and then held left shift key for ages to boot into Safe Mode. This took about 10-15 minutes.
3. I opened Terminal in /Applications/Utilities.
4. Since I had hidden updates in a vain attempt to stop the borked shutdown process, I had to re-enable them. So I typed:
softwareupdate --reset-ignored
5. To list the available updates and check that the 2018-001 was still there, I typed:
softwareupdate -l
which responded after quite a pause with
Software Update Tool
Finding available software
Software Update found the following new or updated software:
* Security Update 2018-001-
Security Update 2018-001 ( ), 506017K [recommended] [restart]
6. Thanks to 9to5Mac's article here:
https://9to5mac.com/2017/07/20/how-to-update-mac-using-terminal/
I typed the following variation on the author's last suggestion to run a shell script that would install all updates (the -ia switches) in verbose mode (if it works or not), then if successful reboot:
sudo sh -c "softwareupdate -ia --verbose && reboot"
Make sure that the straight quotes "" do not copy over as curly /smart quotes “” because if they do, that line won't work.
Because you are requesting superuser access, you need to enter the Administrator's password at a prompt:
Password:
Which will show a key symbol. It will not echo any characters you type, nor even bullets to represent each character. Type slowly and carefully. It will let you know if the password is incorrectly typed, in which case try again. In my case I got the following response:
Software Update Tool
Finding available software
Downloaded Security Update 2018-001
Installing Security Update 2018-001
I briefly also got some warning about the need to add a --restart option flash on the screen. I think that was in response to the first part of the shell script - i.e. the softwareupdate -ia --verbose part before the &&.
Anyway, the iMac rebooted, this time NOT sticking at a black screen with mouse cursor, but proceeding after a couple of stages to a normal looking boot screen (Apple logo and progress bar) and then a Software Installation / Update screen (sorry, I don't have a picture) and then another progress bar, for the first time with an estimated time (31 minutes).
In fact, the software update installed much quicker than this.
7. Just to be sure, because this one has caused me - and it seems others, too - so much hassle and wasted time, after booting finished and I logged in, I clicked on the Apple logo and selected About This Mac, then clicked again where it displayed 10.13.4 to reveal the build version (17E202).
8. Running App Store did NOT confirm that the update had installled. However, returning to Terminal and typing:
softwareupdate --history
replied with a long list of updates, most recent first, beginning:
Display Name Version Date
------------ ------- ----
Security Update 2018-001 29/04/2018, 18:46:11
The version being blank for this update - adding to the general sloppy vibe of this particular update.
Whew! What a relief. I think I will try to use command line updates in future for a while. Seems these give you more feedback on what's happening than the App Store disaster area.
Please let others know if this helps or not by replying below.