I have a MacBook Pro and cannot get it to work with USB audio interfaces. I have tried using 2 M-Audio products (JamLab and now Fast Track USB) and 1 Line 6 product (Tone Port GX). I downloaded the most recent drivers, I set the preferences properly for sound in the control panel to use the appropriate driver, and set Garage Band's preferences to use the appropriate drivers. When I plug in my guitar to any of the 3 USB ports I either get screeching or nothing. The input level in Garage Band varies from nothing to random erratic blips. My guitar and cables work fine with a regular amp so I know the problem does not seem to lie in that equipment. Is there a known issue with USB audio interfaces and the Mac Book Pro? This is sooooooooooooo frustrating. Any help would be greatly appreciated. Thanks.