For future reference, the backup is not meant to be your only copy of your data...it is meant to be a copy, so if the backup fails, you still have your original data until the external can be replaced.
That being said, if you tried swapping the boards of the HDD yourself, you may have voided your warranty with Hitachi. They also don't warranty data, just the drives, so if you really need what is stored on those drives, and you can't get it working with the freezer trick, you may be looking at thousands of dollars from a recovery company.
Have you tried the drive without the external portion...as in connecting it to a computer as an internal drive?