Using Arduino UNO as a USB to TTL converter for unbricking a Seagate Barracuda 7200.11 Hard Drive
I just did yesterday that job of using Arduino Uno as a TTL to USB converter. 
Just put Arduino in Tristate mode and you don't need any sketch on it. The ATMega8U2 did the hard work!. Just follow these instructions: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1258845741

Thery're for Arduino Mega, but the logic is the same: put to ground the Reset Pin with a wire and the ATMega 328 will go in Tristate mode, so you can use directly its serial pins to communicate with your hard disk.

Only check accurately the ground and Vcc connection. I had a hard work to investigate why characters on the console arrived very dirt. The solution was to disconnect the AC adapter on my notebook and go only with battery. Perhaps the adapter did some intereference on ground between the pc and arduino.