I have the 160 GB Moto HD DVR and I only have about 15 hours of HD programing and it keeps telling me its full... I think its a disk error. Just want to make sure before I go through the hassle of exchanging.

Depends on the compression, but that's not unreasonable

I was just looking at the HD Tivo today and it says it can hold 20 hrs of HD on it's 160 GB hard drive. Here's the link where I read it on the Best Buy website

http://www.bestbuy.com/site/olspage.jsp?skuId=8386999&type=product&id=1180743545252&ref=06&loc=01&ci_src=14110944&ci_sku=8386999

I don't have any personal experience with the HD Tivo, I'm just considering picking one up. Considering the source, that may be a best case estimate for the capacity so I'd say 15 hrs isn't too bad, if not the norm, for that size hard drive.

do you have any regular content saved as well? that will also eat into your free space, obviously.

ive got 750GB with my external HD

Is there anyway to add external storage to a rented unit from cable company?

Grande Cable company - Motorola DCH6416
(160G High Def DVR with eSATA & USB ports)

Motorola DCH6416 is the model I have...

20 hours of HD conttent is not very good. Is this fact or opinion?

I can't give you exact numbers but we recorded 8-9 hours of Lord of the Rings off of the HDNET channel and something like 1/2 of our storage space was gone in one afternoon.

It seems like 1 hour of SD is about 1% and 1 hour of HD is about 5%, with the 160GB drive on the 6416. (Just did the math - 5%/HD-hr = 20hrs)

Given that - your 20 hours seems very likely.
HD is just plain ravenous with space.

I normally try to catch most live shows in HD, and record them simultaneously in SD in case I miss them.

I do try to record things off the DVR over the Video Out cables to an OLD DVD-Recorder to clean up space yet "keep" a copy. Quality is not all that bad.