I will add to Caraibes post another possibility. For some strange reason, (poor BIOS design), sometimes the on-board fake RAID will be assigned its cache RAM even if it isn't being used.
If there is a BIOS firmware upgrade available from your mobo maker I would consider flashing it.
Insanity: Doing the same thing over and over and each time expecting a different result.
Democracy is 2 wolves and a lamb voting on the menu. Liberty is an armed lamb protesting the electoral outcome. A Republic negates the need for an armed protest.