My external hard drive :-)

I throw a lot of data around. I like backups. The difference between USB2 and USB3 will be non-trivial for me.

Seagate has a $99 ThunderBolt dock available but figure an extra $50 for a cable to go with it. It's designed for their GoFlex enclosures but I think it works fine with a bare drive too.

cheap extra hard drive dock would be first

http://www.amazon.com/Thermaltake-ST0022U-Plastic-Docking-St...

all the thunderbolt ones are very expensive.

Not sure I'd use a Thermaltake product with mixed reviews as an example here...