I added the extremely fast (near SSD speeds) 256gb Corsair Flash Voyager GTX USB 3.0 256GB (CMFVYGTX3-256GB). Linus did a recent review of this extremely fast USB drive here:
Performance:
Transfer 2.44gb of 955kb photos: 38s read/115.5s write
Transfer 1gb video file: near instant read/12.4s write
Transfer 5.8gb video file: 13s read/67.2s write
I have the Corsair Voyager Go. Love it and it works like a charm. Nice to have 1 thumb drive I can carry music and movies on that plugs into my computer and my phone/tablets.