@Tom-Elliott Thanks for your detailed response. I can understand the problem well. On desktops, you can simply disconnect the HDDs if necessary. However, the day before yesterday, I had a laptop with two NVMe drives, where that would be very inconvenient. There, the installation also went to the second NVMe instead of the first.
The hint about funcs.sh is extremely valuable. I had already come across the line /usr/share/fog/lib/funcs.sh in the file fog.custominstall. When I wanted to look at the file, I noticed that this directory does not exist in my installation. I had assumed it was old custom code I had written. Now I know that’s not the case. I will download the file from GitHub and place it on the system, and I’m quite confident that this will solve the problem, at least initially.
The system was freshly installed at the end of August with version 1.5.10.1600, so the error must have occurred there.