While hd dvr cable boxes have lots of connectors many of them are not active, this is done by the cable company. I have heard that on some that the usb ports may be active however there is no easy way to tell without actually testing them. I think in most cases the cable company does not want you to be able to do this as then you could fill up the cable box dvr then plug in a hard drive which then you could record on and then plug that drive into your computer and actually be able to get hd recording off from non clear qam channels, mostly my own speculation on that but it would follow how they seem to behave.
“I'm the master of low expectations.” - GWB, 2003