70MByte/sec is probably about right for a portable external hard drive, especially if it's accessing the inner cylinders. The spec sheet for the 1TB WD Scorpio Blue drive (which is a fair guess for what's inside your WD Passport enclosure) shows a max sustained transfer rate of 124MByte/sec, which would decrease so something like 60 to 65MByte/sec for the slower innermost tracks.
You have to understand that hard drive speeds are limited by the density and the spin rate of the platters, not by the SATA or the USB connection. (Well, they ARE limited by a USB 2.0 connection, but not by a USB 3.0 connection).
So even though you have a 3Gbit/sec Hard drive or a ~500MByte/sec USB 3.0 connection, the drives themselves are not going to get anywhere close to that speed.