A couple of days ago I explained how M1 Macs have a Fallback Recovery OS, or FrOS, and how you can engage it. What had been puzzling me is that only one of my two M1 Macs would oblige: on my MacBook Pro, I can enter it every time, but my Mac mini stubbornly refuses. Thankfully, now I have been kindly pointed in the right direction, I can explain why this is, and why your M1 Mac may or may not have FrOS.

When you first unbox and start your M1 Mac up, and take it through initial configuration, it won’t have a FrOS. Similarly, if you at any time perform an erase and install of its internal storage, any FrOS there will be removed. When you install the first macOS update which also updates 1 True Recovery (1TR), the old Recovery OS then becomes the new FrOS, and the new Recovery OS is 1TR, the primary rOS.

This makes the FrOS even better than a copy of the current primary rOS. If anything were to go wrong during the update, you should still be able to boot in FrOS and attempt to fix the problem, perhaps by reinstalling macOS. In particular, one problem which it could solve is the situation where the new rOS gets caught in a boot panic loop: in that case, shut down and try starting up in FrOS.

I have already stated the one big difference between FrOS and the primary rOS, and its only real limitation: you can’t set the system security state (BootPolicy) using Startup Security Utility. Apple explains that this is because “LLB [Low-Level Bootloader] doesn’t lock an indication into the Boot Progress Register saying it is going into recoveryOS”.

Because of that limitation, and the fact that it’s not the current rOS, Apple doesn’t count FrOS as being part of 1TR as such, but as a fallback it seems ideal.

To tell whether your M1 Mac has FrOS available, you don’t have to try any gymnastics with its Power button, to see whether Startup Security Utility works. This is because it’s one of the volumes whose presence is recorded in the iSCPreboot volume in /System/Volumes.

In the Finder, you can check whether it has been created and enabled by browsing from Macintosh HD > System > Volumes > iSCPreboot. There look for a folder named with the UUID C8858560-55AC-400F-BBB9-C9220A8DAC0D. That’s almost certainly going to be the only volume starting with C88. If you see that folder, look for a LocalPolicy folder inside that, and an img4 document within that. If they’re all present, then FrOS should be fully functional. If there’s no img4 file, then it won’t be available until the next update moves the current primary rOS to become the FrOS.

At the command line, simply type
ls /System/Volumes/iSCPreboot/C8858560-55AC-400F-BBB9-C9220A8DAC0D/LocalPolicy/
If that returns nothing, indicating that folder is empty, then there’s no FrOS yet; if you see a long file name with the extension .img4, then FrOS should be good to go when you need it.

I am extremely grateful to @NikolajSchlej for explanation and pointing me in the right direction.