I've never had luck with live TV pause. Any time I've tried to use it, I'm able to play my paused program for a few minutes and then it inexplicably jumps forward to the actual "live" tv moment and I'm unable to rewind. When I press rewind, I can see the cache is still there and the images of what I'd like to rewind to (the part of the show I was actually watching) - but no luck.
I've given up on Roku getting this right. The function simply doesn't work. I'm using the recommended USB drive/specs, etc. I'm an IT professional and, IMHO the problem is with Roku.