My problem differs/differed in that the CPU barely moved on the activity monitor while the disk meter spiked irregularly throughout the song. But like You the track count did really seem to matter much. The error message would appear in projects with a lot of track but also in projects with just a few. Anyway, I run my sample libraries, soft synth libraries etc. from external HD via thunderbolt connection (there´s a lot of instruction videos and info to be found on that topic) and there´s never been any problem with that. In my case the problem turned out to be the Lacie HD (a Rugged Mini 7200 rpm) which wasn´t able to stream audio fast enough. When I ran a speed test on that drive the results didn't´t match what´s claimed in the tech specs for that drive. Having said that I know people who are using the very same drive for similar tasks with no problem. My problem were solved by getting another external HD for streaming audio. Though I get the best results by running the audio from the iMacs internal SSD. Active projects are thus now kept and run on the internally and the stored on a larger external HD