pjmelect it's not the HDD. The HDD is a SATA II or III HDD that is in a SATA III -> USB 3 enclosure (I Hope, and not in a USB2 enclosure).
And a Sata II HDD will perform about the same as a Sata III drive when used in a USB 3 enclosure.
Assuming you are using a USB 3 Enclusure:
1) P67 MBs use a 3rd party chipset, Make sure the Driver is the most current one.
2) make sure that your MB Bios Has the Most recent FW.
3) do you have any software that speeds up USB3 ( Asrock Z68 uses a software speed-up program)
4) Did you double check that the BIOS has USB 3 enabled for USB 3 transfer rate.
Now a little on transfer rates:
..This is a sum of two drives so speed is slower that what is listed for a single drive.
..It makes a big diff on Nr of files and size, That is if you ar4e transfering a large numver of RANDOM files from the external drive the speed would be much slower than if you were transfering a single Hugh File.
.. Note Not sure about enclusures, but the SATA III Apricorn -> USB 3 cable appears to Only work good for a specific chipset (an older one). But I think that is unique to it.