Thanks for your replies.
I don't want to use RAID - and it's not up and running anyway.
The BIOS and Windows prompts were probably just triggered when I reset the BIOS to defaults - and maybe RAID was set to Enabled for a few seconds. Anyway, I rebooted and checked every option in the BIOS and made sure that RAID was fully Disabled - and it was.
BUT... I still can't get rid of the BIOS and Windows prompts.