I'm getting the error 0x800F0A12 which is described here.

bcdedit says:

C:\>bcdedit.exe
Der Speicher für die Startkonfigurationsdaten konnte nicht geöffnet werden.
Das System kann die angegebene Datei nicht finden.

(English: Couldn't open the start configuration. Couldn't find the file.)

(off topic: how can I get those messages in English?)

I played around and I assume that is because the system partition C:\ is not on the first BIOS disk.

There are 4 disks in my PC. On one of them (shown as the 4th in Windows drive manager) contains Windows, whereby the system-reserved NTFS partition is the first primary and the second primary is my main Windows system partition. A few more partitions follow with other (non-NTFS) stuff. I was able to set the first two disks offline (via the Windows drive manager). For the 3rd disk, it says that it cannot set the BIOS 0 disk offline.

How can I ignore that and still install SP1?

I don't want to rewire/resetup my disks.