Are you sure its an actual shielded scsi acble? The thin parrallel cables fit but are not shielded like real scsi cables (If I am wrong on that no flame spray please just laugh and move on ). Is it on the correct side of the mac? ( They usually have a picture that shows it not on top or left side of it ). I hope its not your scsi chip as others have mentioned issues here about them going out in other macs, but hopefully your issue is minor.