I've combed the entire BIOS set up, looked in every menue but I can't find that many RAID/SATA related stuff. AHCI isn't in the BIOS. Thigs that might be related:

Enable/disable the OnChip SATA BOOTROM. Default is enabled. Disabling it didn't help.
Enable/disable the OnBoard Promise Control. Here you can choose the operating modes RAID and Onboard IDE Operat. Enabled with the IDE operat is default. Turning it off didn't work.

Disabling both didn't work either. So, I guess I'm making a floppy with the SATA drivers on it…

Edit: And that's when I remembered the box that came with the new SATA-board. When I opened it I found a disk containing the SATA driver… So, now it's working! Thanks guys.

Übereil