Aggregate setup help

[drumzalicious]

Hey everyone. Trying make an aggregate setup on my MacBook Pro using a combination of the inputs in my MBox 2 Pro and the one line-in on the MBP. Problem is I am not getting any audio playing back when running the session off of my external drive. However when running a test session off of the internal HD of my MBP I get audio coming out of my interface. I've gone into the I/O setup and have everything set up the same in both sessions but no luck

One thing I tried was switching the sample rates to match and it gave me audio for about 20 seconds then went quiet and the session was still running