Since your iMac dosen't and will not do USB 3.0, why do you need to know this?
Because I am looking at purchasing another external hard drive and the one I was looking at [1TB RAID 0 (performance stripe)] does not have USB 3, just eSATA, FW800, FW400 and USB 2.0. Then there was another, non RAID, hard drive that has USB 3 in addition to the other connections.
I am trying to decide between the two and just preparing for when I update my desktop in the next couple years.
I looked at Thunderbolt Drives as well as SSD but being a student in college, I can not afford either of those options right now.
There is a way to make any cheap USB2 or USB3 drive work fast with a Thunderbolt "adapter" from Seagate.
I did it for myself. See how http://youtu.be/1Aa3BXmQVpc
THe Seagate "adapter" is not really cheap but with it any Hard disc can be on TB with only a little work.