Drives with 12 GBps interface speed down-train to 6 GBps. Refer below link with details  [URL] 
Im looking at a guy's MD1220 used array.  He has a mix of 6gbs & 12gbs drives installed in it.  I thought this would only work with 6gbps drives.  Does it just decrease the throughput on the 12gbs drives to 6 in this instance?
MD 1220 12gbs Dell Drives