Update - June 2015
This is an update to my original post on External Disc Performance
I recently bought a new USB 3.0 drive - a SanDisk 64GB Cruzer Extreme USB 3.0 Flash Drive - 245 MB/s. I again used HD Tune to check the performance and found it was a lot faster than the generic USB 3.0 drive I already had. I didn't see the 245 MB/s performance, but it was impressive.
I copied one of my Hyper-V machines to the drive and tried it out - it works very well. I have been using it to run a Windows 8.1 Hyper-V machine and VS2008 for Windows Mobile development, and have been very pleased with the performance - I can recommend this drive.
Comparing this to the generic drive - quite a difference - read performance being approximately twice as fast!