One has to ask you first if your model is bought or rented. If bought, then you could pull out the HDD and clone it onto a Seagate SSD (500GB), using the free cloning software called DiscWizard available for download. Another thing to remember is that HDD for video recorders are optimized for speed not precision. It doesn't matter if a few bits are lost on the magnetic platter, you won't see that on the TV screen, however the video style HDD driver will know exactly what to do with the corrupted video frame(s). A HDD or SSD not optimized for video but optimized for number crunching will try to repeat reading the corrupted disc sector before abandoning the chunk of data. However given that SSD have good track records for data integrity and have the speed to make up for any reading delays, this might not be a problem. A 500 GB Seagate SSD should cost you under $100 and if it doesn't work, you can always recycle it into your laptop or desktop PC.
P.S. 10 years ago, there was a small movement of users publishing ways to swap 160GB HDD for 320GB in Cisco brand PVR. In some cases cloning was not necessary as the firmware downloaded a fresh copy of the OS.