Yes there is.
The easiest way is to purchase and install a PVR Extender. This is a plug and play device, as soon as you connect it, it automatically sets itself up and immdieately gives you more space.
The harder way is to physically open your pvr, and swap out the current hard drive with a larger once (do not attempt if you rent the pvr from the cable company). You can search the forums or google hard drive replacement in pvr to get more info.
You can buy a PVR extender & use that if you like but you are not allowed to open the PVR box & changed the hard drive from the inside as that will void the warranty. The PVR extender just plugs into the USB & its a plug & play thing. That is allowed.