I just tested an 32GB USB 3.0 stick (A DATA 180Mb read / 50Mb write) with no problem in preview or render (Vegas 11/64).
32 GB is enough for at least 2 hour of 1080 AVCHD files from one cam in a multiple cam project and it is faster than a common harddisk.
I can recommend it for this purpose as an alternative to the expensive harddisks nowadays.
/Aje