Modern platter or rotational hard drives don't saturate even the 1.5 Mbps SATA first gen spec without extra help (RAID, very fast spindle speeds), that's roughly 150 MB/sec. Most hard drives barely touch 120MB/sec even when they're the faster, new ones. SATA 3 offers more of a ceiling than 1.5, as does 6.
The bottom line is that the hard drive likely isn't slow because of the interface, there's something else going on with your system or you bought a crummy hard drive
You've got a program or something in the background that is throttling it. You need to check your system for viruses/malware/proper drivers before you do much else.