My ibook slowed down significantly over the last few days and now simply refuses to start at all.. i've tried the single user terminal disk check but it doesn't either find a problem nor tell me the disk is intact.. something about a BUS error.. i can't start in safe mode and now all i get after the tone is a grey screen.. not even the macintosh logo and loading gear start.. seems farily terminal/expensive to me but any advice for someone more in the know would be much appreciated.. james