Two days ago my iMac 20" 2.16 (late 2006) began to slow dramatically and then froze up altogether in finder and then wouldn't restart. I had recently erased the drive with Disk Utility and restored using SuperDuper in an attempt to clean and speed the computer up, so I thought this might be the problem. I ran Disk Utility and was told that the drive had an "Invalid Node Structure" and needed to be repaired. Disk Utility could not do it, and others on this forum and elsewhere recommended Disk Warrior. Disk Utility at one point recommended I erase drive and reinstall system software. Rather than using Disk Warrior, I erased the drive again (did not zero it out though) and did a fresh install of OS X 10.6. I used Migration Assistant and Time Machine to restore my files (I am very well backed up, no worries here). The machine was running beautifully, faster than it had since it was new. Applications were loading quickly, everything was fine. Then all of a sudden - while I was editing bookmarks in Firefox - the spinning wheel of death appeared on the finder, which was not responding. I managed to restart (took about 3 minutes), but it wouldn't boot up. I then booted from my cloned external drive, ran Disk Utility, and there is was again, "Invalid Node Structure". All of the data on the drive is still easily accessible when running off the external drive. So, the question is, should I spend $100 on Disk Warrior now or $200 on a new internal drive (go from 250 to 500MB) w/labor (I am not doing this myself - it's a real headache, from all videos I've seen, to replace internal drives in iMacs). Or should I erase using "zero out" and try one more install? Or maybe just run off the external drive? Any ideas? Do you think this is a hardware issue or logic issue?