Here is what it says about the HR34 on DirecTV's website:
Introducing the revolutionary Home Media Center HD DVR. If you've ever wanted to put an end to recording conflicts and deleting your favorite shows to make room for new ones, here's the receiver you've been waiting for. The Home Media Center HD DVR lets you record five shows at once, store up to 1600 shows* in SD — and watch them in any room of your house. You won't find anything like it with cable or DISH Network. It's available only with DIRECTV.
The Home Media Center HD DVR gives you up to 800 hours of storage space in SD, 200 hours in HD, so you don't have to delete shows you want to keep to make room for new ones.
*Storage space refers to 30 minute programs recorded in SD. Actual recording capacity varies based on type of programming being recorded.
Where do they advertise a specific amount of space? It is always stated as "up to" or "varies".
Add to all that, we've never been able to determine a rock solid answer to the question of how many hours of recordings any HDD will take. It seems to depend on the content more than the time it takes to record it.
When I first got my 20-700s working, I filled up a stock 300GB internal drive with almost 50 hours of HD MPEG-2 recordings. I think they were advertising 20 hours of HD recordings at the time. It's all about the content.