In order to use the Intel RST you have to use a SSD of 64gb or smaller with a minimum of 18.2gb. The whole purpose behind the Intel RST is to have your hdd as a single or in raid to have SSD like performance. Since you have a 120gb SSD it makes more sense to put your OS onto the SSD and have your hdd as storage. A 120gb SSD is big enough for the OS , a couple of games and some apps. This way you don't have to play around with a raid setup which sence it depends on both drives to be functioning to use and if one fails you lose your ability to uise the computer. Having a single drive being backedup makes better sense.