Uhh look at you mobo manual. It should tell you the Sata cable order (Sata 1, Sata 2, Sata 3, etc.)
You want the load order to be lowest to highest. So in your case, you want the 120gb plugged into the Sata 1 port as its capacity is lower than the 250gb which you will plug into the Sata 2 port.
Hopefully that works.
If your OS is on the 250gb go into your BIOS and set it to boot off of that hard drive if its on the SSD you should be good.