I just recently picked up an original Mbox to try out Pro Tools. I am running 7.4 on an Intel iMac w/ 2GB RAM, OSX 10.4.11. The issue I am having is that as soon as I start recording, I get the "CPU usage is holding off USB audio. Remove some plug-ins, delete some tracks, or increase the 'H/W Buffer Size' in the Playback Engine dialog. (-9136)." This happens when recording even 1 track with no plugins, after between 1-30 seconds of recording. The interesting thing is that the activity monitor only shows about 5-10% CPU usage. I have enlarged the HW buffer to the largest setting, but it has made no difference. I also tried on my intel macbook and had the same issue. Anybody else ran into this problem?