I think you guys ought to consider what flash drives are fast because some flash drives are downright low as 2MB/s.
The Xbox 360 USB 2.0 interface goes up to 35MB/s out of the USB 2.0 general 60MB/s limit for PCs so it's even more of a bottleneck for the console restriction than the interface limit itself. But still, you guys mentioned 15MB/s is mandatory for flash drives since that is inherently the Xbox 360 DVD Drive speed. Getting a flash drive that even goes up to that is a bit difficult in and of itself.
I was just lucky when I bought my 32GB Sandisk USB 3.0 drive from walmart and it just so happens to write at 15MB/s on my laptop 2.0 port. Usually 3.0 flash drives make way better use of the 2.0 speeds.
I'd recommend the HyperX 3.0 since that actually achieves 30MB/s on 2.0 ports which is nearly the Xbox 360 USB limit.
