First, reassure yourself the drive is OK. The SMART verification in Disk Utility, TechTool Pro, SMART Reporter, and others is not very good--the warning thresholds are set so high the disk is usually toast by the time you have a clue. My favorite app for this is SMART Utility, available on versiontracker.com. It's shareware but has about four free launches before you have to pay. Download it and run it. It accesses the SMART attributes on an internal drive (sadly, not on most Firewire or USB connections) and will warn you if your drive has failed or if it's in the process of failing, which has saved my clients many a time.