Never had a USB HD work reliably
I've had a few USB hard drives - both 2.5" and 3.5" ones, always been a pain, numerous times XP/Vista wanted to format them before it would look at them, even though the drive had been used the day before! Reading from USB drives not so much a problem, but writing to them has always been hit and miss.
Eventually I bought a d2 quadra (has USB2/FW 400/800 and eSata and comes with all the cables) and an eSata express card for the laptop. Has worked *every* time, its quick, and great for video editing. USB3 drives might actually work properly, but until thats proven, its eSata for me for external drives every time.