I have a MBP with a 5400 speed drive, and I can play all of the demos, even unfreezing the tracks. If you get the message only once at first playback, that's a known issue with Logic and there are tons of threads concerning that. Just consider it an annoyance and and learn to live with it until there is a fix. If you're getting them continuously, there is another problem.
Try trashing your preferences, that has worked for some:
http://discussions.apple.com/thread.jspa?threadID=1175006&tstart=0
If that doesn't work try searching for more workarounds. A lot of people here have explored this issue.
Good Luck!