Hope I'm doing this right, I never posted before.
"They will all work" is only partly correct. I have a Seagate Free Agent 1T, which as I've learned is really 2 500 GB drives wired together (which I suspect is the issue for me). I had the "power save" problem others have talked about, but used the drive so infrequently I just unplugged it and plugged it back in when I needed it. Aside from it powering down after a few hours, no problems.
Then, a few weeks ago, as soon as I surpassed 500 GB saved to the drive, The DVR stopped being able to read the drive. Or, more accurately, what was stored on the drive. It can still read the drive (it shows how may GB remain), it can still write to the drive, but it shows 0 GB of shows saved whenever I try to retrieve them. Dish technical assistance, "yeah, we're having a problem with that drive, no solution" (thanks for the warning); Seagate, well, you can guess, less helpful than that (at least the DISH tech was nice about it). So now I've got a pile of junk with 6 months of movies and select shows saved on it that no one call tell me how to get to.
So, stay away from the Seagate 1TB Free Agent. I'm sure it's a fine external drive for computer back-up, but it's a disaster waiting to happen for DVR storage (or at least DISH external storage).
And, if anyone can direct me to a hard drive that they have actually used, that has a demonstrated history of reliability (I prefer 1 TB but will take anything at this point), I'd be very obliged.