CPU holding off USB audio (AAE-6104)

Hi I'm new to the forum, but have been a Pro Tools user for a number of years.



I recently upgraded my set up but am continually getting this error message.



CPU is holding off USB audio remove some plug-ins, delete some tracks, or increase "H/W Buffer Size" in the play back engine dialog (AAE-6104)



I have followed all the steps I've been able to find on this forum and via google searching on optimising my macs performance.



My set up is



Pro Tools 11.1.3



MacBook Pro

2GHz Intel Core i7

8GB 1600 MHz DDR3

OS X 10.9.2



mBox 3G audio interface (with USB cable supplied and tried both USB 3.0 ports)



I'm using a LaCie d2 Quadra Hard Drive connected via Thunderbolt to FireWire 800 and working from pro tools sessions on the hard drive.



I have read the help us help you section,



My Pro Tools sessions are 24bit 48kHz



looking at the System Usage Window in Pro Tools



CPU total is around 9% all 8 bars are between 2 and 5%

Disk is 15% and Memory 20%



there are a couple of things I think it might be, but could be wrong...



I bought the mBox with Pro tools express and an upgrade to Pro Tools 11, it's sort of left the Avid folder in applications a little messy/disorganised with double of a few things, and I haven't deleted pro tools express didn't want too incase I messed up the upgrade? and didn't know which folders related to PT 11 or PT ex



I have also read on the forum that it could be the mBox having troubles with the USB 3.0 ports? I have ordered a D-Link DUB-H7 7 - Port USB 2.0 Hub External which I saw had solved a previous forum users problems which I'm waiting for delivery of.



I can't seem to find anywhere in the instruction manual or online what rpm the LaCie hard drive is. Could it be the wrong rpm? could it be the hard drive is also used to store my iTunes and iPhoto libraries even though these applications are closed when I'm running Pro Tools?



I have a live acoustic recording session coming up in under two weeks is there anything settings or preferences wise I could adjust that I'm missing? Is something I'm using not compatible somewhere?



I hope I've listed everything you need to help me.