I found the SanDisk Ultra Flair USB3 Flash Drives worked well but I'm constantly re-evaluating this. The drive I used was only very infrequently used for recording and a few weeks ago started having issues. I checked the drive on a PC and found the write speeds had dropped off a cliff. SanDisk replaced the drive free of charge (5yr warranty) however that got me trying a SanDisk USB2 drive which was basically OK although there were occasions of the odd stutter when recording and playing back at the same time.
Although outright speed isn't supposed to be a requirement (I've seen this documented by Sony somewhere) I suspect that a drives ability to maintain a constant throughput of data is necessary and this is where many drives fall down.
In the end I settled on using an old USB2 320Gb traditional drive I had and its been flawless.
