The very first two-part installer for High Sierra was pulled from the App Store when it failed to work for many of those trying to upgrade. The second monolithic installer has also had its problems, with users suffering frozen installations, and most commonly failed migration.

When Apple released the High Sierra Supplemental Update, it claimed to fix some, if not all, of those installer problems: Apple’s perfunctory release notes claim that the update “Improves installer robustness”. Perhaps I am reading too much into the fact that installer starts with a lower-case i, indicating that it refers to the High Sierra installer, not the Installer app bundled with macOS.

I have now tried two versions of the Supplemental Update – that provided in the App Store, and the standalone installer package provided at that link – and am still puzzled as to what exactly they are intended to do.

If you download the Supplemental Update from the App Store, it runs, updates the current High Sierra installation, quits, and deletes itself. It doesn’t appear to be designed to be much more flexible than that.

So I downloaded the separate installer package, and tried to get it to perform three important tasks:

• to update an external APFS SSD containing an installation of High Sierra, which had not yet been updated,
• to update a bootable USB memory stick containing the first monolithic High Sierra installer,
• to update the first version of the monolithic High Sierra installer app.

This installer package would not even attempt the third, and did not recognise that app as something which it could update. It did allow me to select the bootable USB memory stick and the external SSD for update, and started trying to update them. However, it fairly quickly decided that it couldn’t help with either, and bailed out with an error. Ironically, the error message hasn’t been customised to refer to Apple, but just directs the user to “contact the software manufacturer for assistance.”

Perhaps if I were to start up from the external SSD, it might be of greater help. However, the only way to create an updated bootable USB memory stick appears to be to make a new one, using the new version of the whole High Sierra installer available from the App Store.

The only snag there is that, because I have already downloaded the first monolithic installer, and because Apple does not include High Sierra in the Purchase tab, the App Store doesn’t offer me the new installer as a download at all, but just offers to open it. Perhaps I need to delete the original installer from Applications to allow the new version to be downloaded…

This seems to be a serious, and completely avoidable, mess.