Thunderbolt on external HDD's is nothing but a marketing scheme. Most mechanical HDD's don't even saturate SATA I speeds (1.5Gbps). In fact, my WD Caviar Black drive (which is typically faster than what is used in most external hard drives) gives me sequential read/writes at just under 100 MBps. That is 800Mbps (please note the difference between bits and Bytes), which is the top speed of Firewire. Thunderbolt has a top speed of 10Gbps (that's 1.25 GB's per second). You honestly will probably not see a bit of difference between your Firewire drive and your thunderbolt drive.