I would install the ResScan software and see how it reacts with the card. The links for how to do that are located http://www.apneaboard.com/forums/Thread-...d=99#pid99.
Or, I would stop by my friendly camera store and purchase a new 2-4 GB SDCard (nothing bigger, nothing other than "SD," or you'll make the Resmed unhappy) and try using it.
But zero ahi seem to me to be one of two things: You are at zero (not impossible, I hit zero about 3 in 5 nights) or the ResMed is not working right. Using the dials on the Resmed, does it show zero events?