Why is my SATA HD only transfering at 53MB/Sec to another SATA HD?

I’m transferring data from one HD to another HD. Both are internal, and both are SATA. The transfer rate is roughly 50ish MB/sec. I understood the data transfer rate for SATA 1.5 Gbits/sec to be around 150MB/sec.. give or take, and for SATA 3.0 Gbits/sec to be around 300MB/sec.. give or take. And these ports are definitely 3.0 Gb/sec.. so what factors are the most likely culprits for the slower transfer rate?