Just to clarify here, this has been asked many times before. You can hookup an external HD to the Comcast DVR, but the box won't recognize or do anything with it because the eSata and USB ports are disabled.
So the only way to transfer recordings off (without lossy conversion) is to use firewire to a PC or an old DVHS recorder. You can also buy a Hauppauge HD-PVR device to record from the component outputs at some small loss in conversion, but I think that is for XP and Linux PCs only.
You'll be wanting to check out this thread
for info on how to do firewire with a Mac.