I hooked up the hard drive from my motorola DVR to a pc running Linux. At first it appears to be unformatted, but if you use partition magic or the like, you find 3 partitions. The first 2 seem to hold guide and programming information, but the last is filled with recorded programming. The programs are .tsp files, i think, and work well in linux for viewing. I have found no way to convert them into something windows will understand and I have no DVD burner on my Linux machine. This is where we are until the cable companies allow us to pull the files off over the useless firewire or even more useless USB. It all has to do with copy protction and "lossless" transfers. If space is truely your only concern, you can attach a mac harddrive (like one meant to increase the size of the mac mini) to the firewire port and you will have more space. Also an iPod will increase your DVR's harddrive capacity by whatever free space you have on it when you plug it into the firewire port.
Good luck all!