A lot of Cable Providers are using the Scientific Atlanta series of HD DVR's. The most recent models are the 9000HD and the 9300HD (although I've seen the 3225HD still given out).
The 9xxx series (in San Diego) runs the Pioneer Passport software (on both Cox and Time Warner). There are two versions of Passport in use:
Passport -- Normal Cable Box firmware, it does have DVR functionality, but it's pretty limited. It's like having a built in VCR.
Passport Echo -- Newer Cable Box firmware. It's designed to run on DVR boxes with more advanced DVR functionality, such as Series Manager and things of that nature.
On the SA9300HD boxes, there are in fact external SATA and Firewire hookups for external storage. Unfortunately, it's up to the cable provider to ENABLE these ports via the firmware.
In San Diego for example, it's enabled on SA9300HD's running Passport on Cox Cable, where as Time Warner disabled it on it's SA9300HD's running Passport Echo.
It's different from region to region and it's up to your cable provider to enable this feature. So I'd first call your cable company and verify that this feature is active in your area on their new DVR hardware.
Also, the newer 9300HD mode's come with an internal Western Digital 160GB SATA hard drive, which has plenty of space for HD and NON HD content.
