Update 2021-11-01: It seems this error is quite common, and at this time, the only solution confirmed to be working over on StackExchange and MacRumors is the one that Worked for me, too.

I’ve spent a few days trying to install macOS Monterey on my late 2014 Mac Mini. In order for other people to save some time trying various general purpose advice, I summarize my findings here. In case you just want to know how to fix it, jump to the solution section at the end of this article. If you want to understand what is going on, keep reading.

The Error Message

Running the macOS Monterey installer, after a few minutes, and right before the first reboot („closing programs and preparing reboot“), an error message was shown (German: „Beim Vorbereiten der Installation ist ein Fehler aufgetreten. Versuche, das Programm erneut auszuführen.“, English: „An error occurred while preparing the installation. Try running this application again.“).

While trying many things in order to fix the issue (see below), I came across another error message:

osinstallersetupd[313]: Forcing MSU springboard for target: disk1s1 (has EFI without NVMe update support)

What did NOT Work

All of the following approaches did not work and led to the same result:

• Running the installer from „System Settings“
• Copying the installer bundle from another Mac
• Checking and repairing the disk using Disk Utility
• Creating a new admin user account and installing from there
• Using System Restore (⌘ + R)
• Using Internet System Restore (⌘ + ⌥ + R)
• Using a bootable installer USB drive

The Reason

A few days later, a web search finally brought up a thread in Apple’s support forum, which contained an explanation as well as a proposed solution. The argument goes like this:

• The issue is most likely caused by an outdated EFI / boot ROM
• Boot ROM updates are not provided as standalone updates, but are included in macOS installers
• The Monterey installer needs a new boot ROM in order to run successfully
• Apple just updates the boot ROM if an original, Apple-produced system drive is present
• On Macs with third-party system drives, the boot ROM update fails, causing the error message

The Solution

In order to verify that your Mac has an outdated boot ROM, you can check your current version – either using an app such as SilentKngiht, or by running the following Terminal command and checking the result against this list.

/usr/libexec/firmwarecheckers/eficheck/eficheck --integrity-check

If your boot ROM is indeed outdated, and you are lucky to have the original Apple hard drive or SSD around, you can upgrade to macOS Monterey by following these steps:

1. Remove your current SSD, put in the original SSD
2. Install or update to Monterey on that drive
3. Verify you got the latest boot ROM
4. Power down, swap your main SSD back in
5. Install Monterey on your main SSD

For me, this completely fixed the issue, and I now run macOS Monterey on my Mac Mini with its third-party SSD.

Further Reading

Here is another resource I discovered during my quest (thanks to @AppleSupport): If an error occurred while updating or installing macOS