[Mike Warmels] “Both the same.
“
That’s not particularly fast, I wouldn’t expect to have smooth playback whilst editing in “better quality” mode with a USB3 drive. I do think that FCPX needs faster drives, but I’ve also found exactly the same thing with Premiere Pro CC, so it’s possibly not just an Apple plot. I don’t know how Avid handles playback so I can’t explain why it seems to perform better.
I understand your frustration, especially since you have a new Mac Pro, but to be honest I wouldn’t even consider doing day to day editing on USB3 drives, the bare minimum I use is single drive Thunderbolt.