I respectfully disagree, having seen how the internal drives are mounted within all 3 HR24 units.
The foam will indeed aid the vibration sourced from the hard disk drive and dissipated within the mounting housing and through the HD DVR itself thereafter. If that were not so, there would be little reason to even have the foam, let alone the rubber grommet mounts. The HR24 drives are now so quiet that the foam is virtually irrelevant, as far as hard drive noise alone.
For that very reason, installing a "different" drive without the foam risks introducing the same vibration issues.
I think we've gotten to the point of splitting hairs.
What percentage of all the world's hard drives are protected against vibrations using foam? Very few. And I don't think there are a whole lot of hard drive failures that would have been avoided by using foam. So I think these "risks" from vibration issues are a non-issue, at least in a home-based DVR application. I mean hard drives aren't so fragile that they are susceptible to damage from their own vibrations...
Maybe the foam gives some minimal protection against failure but I think the main purpose is noise dampening.
In any case, with the WD 2TB Green drive I think the foam is a bad idea because of the amount of pressure it physically puts against the top of the drive. This cannot be good. If I was using a hard drive that is thinner and doesn't compress the foam so much I'd keep it in there. But with foam or without foam I don't think it matters much at the end of the day.