Hallo,
one thing that could be happening is that the disks still contain the raid signature. Most of the onboard raid is purely a software thing and for many Linux knows the signature and supports the same raid format in the dm-raid modules. Weather the raid support in the bios is on or off doesn't matter here as Linux will always see the raw disks and only the signature on the disk is relevant.
On the other hand the installer doesn't know about dm-raid and can't wipe the raid signature. It would only see the disks as being "in use".
Please run "cat /proc/partitions" and "ls -lh /dev/mapper" to see if there is a raid up and running. Removing the raid signature on the disks would solve the problem then.
Regards, Goswin von Brederlow