Tried updating the Mem’s Shed iMac to macOS 10.13.6 last week but the Combo Updater I’d put on an ext drive was faulty & somehow incomplete. I did manage to get it to accept iTunes 12.8 though.
Today I used my newly-created TechTool Pro-To-Go drive which is on an 128 GB ext SSD to boot the Mac. I then copied Safari from the TTP drive to the Applications folder on the iMac. Next I opened the Combo Updater & told it to install to the iMac itself rather than the external drive. This it did & when it restarted, the process was a bit slower than usual but still faster than doing an update from the same drive.
The the Software Update section of the Mac App Store sat seeming to do bugger all. it was insisting that the Safari, OS & iTunes updates package needed applying, but I felt it shouldn’t be the case.
The internet at the Shed is supplied via a pocket WiFi on a 4G network that’s just not as fast or extensive as the network I have my phones connected to.
So I started internet hotspot on the Lumia 1520, it was a significantly faster connection & the App Store was then almost instantly able to deermine than the right updates had been done.