As for vibration, yeah, lots of factors. Obviously the better quality the storage array used generally the better the build quality – and the lower the vibration and other issues. Dirt cheap arrays just cram as many drives in as possible. Quality systems use isolation mounts, etc.

But strange things happen. One network product I’ve worked on had an issue with a batch of hot swappable power supplies that had unusually high fan vibrations, and those vibrations were killing drives in the units. Replacing the problem batch resolved the issue. It just goes to show that complex systems have a lot of possible failure modes.

Don’t be too sure about the RAID thing.  I know people who worked on the product and testified in the Cablevision case and certainly during the trial every single step in the process was examined in great detail, including even things like copies IN MEMORY.  And yes the original system built by Arroyo to Cablevision’s specifications was very carefully designed to be as close to a normal DVR as possible, just with a network between the hard drive and the STB in the home. 

Certainly I doubt any of the follow-on products, or even Cablevision’s deployed nDVR product, will be designed with this level of care.  Certainly I would expect them to slowly but surely creep towards an efficient design where multiple user’s recordings could be shared.  Comcast for example is planning to support both shared and unshared recordings, the shared for networks they own or negotiate agreements with (hey, we’ll put newer ads on those DVRed shows, or suppress fast-forwarding over commercials someday, or something, but you have to let us share the recordings we’re going to make anyway…).

Yes this works better for a cable MSO than satellite.  As you say you expect over time that DVR users tend to watch a lot of stuff from their recordings rather than live, and all of those recordings will be “VOD” like, in that they can’t be shared with anybody else.  Even if we both started playback at exactly the same second, as soon as I press pause when you don’t, we’re suddenly watching different streams. 

And just one final comment on data-centers… as you know disk drives are tremendously susceptible to vibration and lots of the drives in a modern data center are packed very tightly together, which of course results in lots of vibration.  For example it used to take a 3RU rack mount chassis to handle 12 3.5 inch drives, but now that everybody is moving to vertical mounting they get even higher densities.  We’ve had drives fail in such systems at a much higher rate than you might expect, and little things like rubber grommets and such become critical.  So they MIGHT have more reliability than that drive would have in your house, but they might not.  Depends on a lot of factors.