I'd be careful about connecting anything to your Comcast cable box that they don't say you can. It's a rental, not one you own.
Does your box have a USB port on it? If there is, have you asked Comcast what it's for? Have you asked Comcast if they can give you a box with a bigger drive?
With this being a box you're just renting, I'd deal with the company that owns the box before I'd ask on the forum. Or at least don't act on anything said on the forum without checking with Comcast first.
No sidewalks with beef, please.