Hi
I have a server with an Intel S875WP1-E motherboard and 4 HD of 400Gb each.
If i use the raid utility from the motherboard it doesn't appear during disk detection, so i disabled it. I've rebooted and launch installer from the DVD and it displays correctly 4 HD. I've configured them, installed the OS and installed GRUB in MBR (grub install /dev/sda /dev/sdb /dev/sdc /dev/sdd).
But when i reboot it asks me to switch for a correct system disk.

I've look into BIOS configuration. If i make a raid array using the bios utility, i can choose any array in boot device priority, but if there is no array defined, i can't select anythig (but the DVD drive).

What can i do?

Tough one!
Looks like your bios is only recognizing the disk with the controller enabled. The linux-kernel when loaded sees them all as normal sata-disks. You could try and set them up on the controller as 4 raid0 devices. Dont know if the controller supports this.
Take good care here, because this might destroy your data on the disks, but I suppose this is a new install.
Otherwise try and install grub on a floppy or cdrom and boot it that way. Try supergrubdisk first to find out if can get grub to boot it at all.
Good luck!