I have the flashing finder icon problem. When I boot from DVD, I cannot choose the disk as Startup Disk. However, it appears in Disk Utility. When I run Repair Disk I can see the disk, and that it has about 130GB used, but Repair Disk takes literally about four seconds -- a suspiciously short amount of time. By contrast, Repair Disk on the external takes about three minutes.
What is the best explanation for this?