One of the issues that's been reported here by me as well as several others is that when you need to switch your sample rate from one session to another, say going from 48k to 44.1k, you can't just open Mix Control, switch it, and then open your PT session and off you go. Most of the time PT just won't respond and you end up having to not just re-boot PT, but your computer as well...sometimes twice. I have personally gone through every system check possible with Focusrite on this issue with no resolution.
Well, at long last, I think I have found a workaround that at least makes it easier and avoids all the re-booting. This works for my Windows 7 based system...I don't know if there would be a corresponding workaround for Macs (or if MAc users even have the issue at all). But if this HAS been an issue for you, then here's a workaround.
First, go download
ASIO4ALL, which allows to use your PC or MAC itself as the interface. Set that up as an additional interface in your Pro Tools.
Now, let's say you want to go from a session using 44.1k sample rate to one at 48k sample rate. Do the following.
1. Close your current session
2. Exit Pro Tools
3. Open Mix Control
4. Set sample rate in MC to the new rate - 48k in this example
5. open PT...but NOT the session
6. Before opening the new session, change your interface in PT from your Focusrite to the ASIO4ALL
7. Open your new session
8. Hit play...if it plays, all is well.
9. Now, go back to PT settings again and switch your interface back to your focusrite and you should be fine.
A few steps involved I know, but its better than having to reboot your entire system and once you've got the process, it takes all of about 2 min from start to finish.
Until Focusrite and Avid can solve this one, this is a reasonable work around. Hope this helps anyone who has this issue. Good luck.