I had a very similar problem with an external disk I had been using with my intel iMac. I had a full backup on the drive (using SuperDuper! - great program btw), but after a reboot, the drive was suddenly no longer accessible. I had intentionally wiped the iMac's internal drive in the process of trying to install XP since bootcamp was giving me issues, so the backup drive was now the only drive containing all my files. Needless to say, getting back the data was imperative.

I tried a couple of disk recovery applications, but Data Rescue II was the only one able to retrieve anything from the disk, and even then, the files it recovered weren't really files - more a bunch of conglomerations of random blocks on the disk. Nothing usable.

So, since I couldn't afford to send the disk off for professional recovery services, I decided to try to fix it by hand. I started investigating the various partitioning formats, and found that the disk was partitioned using Apple Partition Map. When I viewed the data on the disk in a hex editor, I found that all the entries in the partition table had been shifted by 32 bytes. Very odd.

I spent two days writing a program to understand this partition map, so that I could verify it was intact in preparation for putting it back in the right place, hoping that it would just "work" after that. Then something strange happened.

I went to run my program, to show my girlfriend the difference btw a good reference disk and this oddly shifted one, when, suddenly, it was no longer shifted! How could this be? It had repaired itself! This is extremely odd as I had connected/reconnected, mounted/unmounted this drive many times (as well as rebooting my macbook) in the process of troubleshooting. How could it suddenly just start working normally? I remain perplexed.

The drive is working fine as I type.