It has been reported here, in the past, that some Panasonic DVD recorders can have the hard drive replaced by getting a compatible drive and installing it in the machine. When you power it up, the first time after the new drive is installed, the DVD recorder says that the HDD needs to be formatted, and requests you do so before it can proceed with the power up sequence. Once that is done, the machine works like always. If you put in a drive of higher capacity than the original drive, the DVD recorder will format the newer drive to the same size as the original drive, leaving the additional space unusable.
You didn't post your model number, so there is no way of knowing if this will work for you.