I haven't seen one yet, but why? Why not just use an internal drive, which will have much better read/write/sequential speeds than a flash drive could do (I'm not talking SSD's).
The typical flash drive can read/write usually around 30MB/s max. Most internal hard drives can reach 100MB/s. Sure they have a bit higher access time, but it's measured in ms, so you shouldn't be able to notice a difference.