Were the sata/sata tests done to the same device, or different 3/4tb drives?
I bought four brand new drives (2 x 3TB and 2 x 4TB Seagate, not WD (oops)). I installed one of each in my PC, copied files from smaller drives (5DMkII MOV Video files to the 3TB and JPEG/CR2 to the 4TB), then started to make duplicates on the other two corresponding new drives. Also reformatted 1.5 and 2TB drives, then started copying TV and movies (250MB - 4GB) to them, as well as making backups. I've tried most of the possible combinations, and each one seems to show copying onboard SATA to/from USB 3.0 is much faster than from SATA to SATA.