The SATA connection is like a freeway. Older freeways have a speed limit of 3Gbit/sec, the latest ones have 6Gbit/sec. But a pokey hard drive that tops out at about 1.2Gbit/sec or so isn't going to go any faster on the newer freeway. It's like a bicycle - its top speed is only as good as the cyclist, not the speed limit - and no cyclist can keep up with freeway speeds.
Not even most SSD's can't exceed 3Gbit/sec - only a few of the very fastest ones can. They're the only ones that be able to go as fast as they're able if they're on the new "freeway".