There is AVS Forum discussion of the Digital PCB, a Panasonic's "brain." The hard drive controller board is connected by a ribbon cable to the Digital PCB. (The Digital PCB serves as the DVD Drive controller in 2006 models.)
While Digital PCBs may share "VEP" numbers with Digital PCBs found in other models of the same model year, these Digital PCBs generally have a model-specific factory configuration and feature-set so they may not be swapped between models with different feature sets. Perhaps the hard drive controller boards are also model-specific. (There are also some differences between the Digital PCBs fitted to a single model, e.g., I posted comparative photos of two DMR-ES35V VEP79132 Digital PCBs, one with a ribbon cable connector for the hard drive controller board ribbon cable and another DMR-ES35V VEP79132 Digital PCB without that ribbon cable connector. The DMR-ES35V is a combo recorder not equipped with a hard drive.)
As to hard drive compatibility, at least one AVS Forum member mentioned temporarily swapping a hard drive from a non-functional DMR-E100H (with a non-hard drive related problem) into another E100H and dubbing that hard drive's contents to DVD, after which the original hard drive was returned to the functional E100H. This indicates that there is no machine-specific "encoding" of the hard drive.
There are also AVS Forum discussions related to hard drive replacements/upgrades for Panasonic machines.
While I own many Panasonic DVD recorders none of them are HDD/DVD models. I also own one Philips and two Magnavox HDD/DVD recorders. A few days ago I upgraded one Magnavox with a larger hard drive, a thirty-minute project.