Umm, so your drive used to be hooked up to a firewire-to-ide converter chipset ? I'm not sure how well those chipsets work... it's been a known issue with software based raid controllers that many of them use chipsets where the drive works fine with that controller but you can't just carry the drive to another computer's raid controller card and expect it to work. I wonder if the converter chipsets have the same issue.....
why did you take it out of that external case ? what prevents you from buying that exact same type of enclosure and then putting the drive in the new one (before you mess with the mbr, etc..... oops, too late).
[edit] the most common deskstar problems appear to be related to thermal expansion causing the loss of tracking and/or head crashes into the surface. I'm not sure IBM ever laid out exactly what they were finding in all their RMAs and it was left to internet geeks to discuss and try to figure out. I heard one story where a chip on the breadboard was overheating and someone was able to fix their drive by cooling the chip with an "ice in a can" product and then transplanted circuit boards from a working drive into a non-working one.... it made it work but the starting head positions were off and they couldn't recover their data. Common Deskstar problems seemed to be largely physical and not something software can recover from.