I'm not sure how DIRECTV's version would be different. You would need the drive to be external to move it to a different unit.
1. Dishes implementation is simpler as they only store video on the external drive.
That's how DISH's EHD solution started.
2. All the external drives in use now are encrypted and tied to the individual DVR.
If they can play the content with WHDS, they've already demonstrated the ability to use a foreign key.
3. That tying to a specific DVR means that to go to a whole house key so that any External will work on any DVR in the house would mean wiping any External drive now in use.