Since everyone is busy upgrading their Roamios, and this question came up in the previous thread, I'd like to address it in a separate thread. That way if this thread gets closed, at least the other thread can remain open.
I have actually worked at a company that makes hard drives, as an engineer. I can't publish the proprietary designs because I don't work there anymore. I could also get sued for divulging too much information. So I'm walking a tightrope here. If I'm too vague in some areas, it's because I'm in CYA mode.
In the very beginning (10 years ago), there was a difference between them. It had to do with thermal calibration. Normal drives would have to go offline for a few 100 ms to recalibrate themselves based on the temperature change in the hard drive. That obviously would be bad for a streaming application. But modern techniques make type of thermal calibration unnecessary. Modern drives can recalibrate for temperature on the fly.
Then there was a period where there was no difference between AV drives and green drives except profit margin and a longer warranty.
Before I left, they were actually developing different production lines for the two models of drives. Though mainly because they needed to add another green/AV line to keep up with demand for AV drives, and they just decided to make it a dedicated AV line to shift the AV demand off the other lines. So now there COULD be some differences between them mechanically, but I'm thinking the differences in hardware are trivial. All hard drives are designed more or less the same (except for the RPM and case height), and they all are designed (as best as possible) to run 24/7/365. No hard drive mfr. recommends powering down their hard drives a few hours a day to give them a rest. Even modern OSs are constantly doing some housecleaning, so a green hard drive will constantly be accessed whenever the PC is on.
The firmware differences were also subtle:
Acoustic mgmt: green set to normal, AV set to quiet.
ATAPI 7 streaming support: both had it, technical support provided and advertised only on AV drives. The AV drives had the latest bug fixes, while the green drives were a build or two behind but more stable. It seemed silly to intentionally disable that support in green drives -- it would have created more problems than benefits.
That's it for the differences. It turns out that most consumer PCs also want most of the AV drive features, including fancy read/write techniques to speed things up even more. Intel's RST is an example of this.
Regarding the streaming support (including the "don't report an error" feature), the commands for those are different than the typical hard drive commands. They are "extensions" in ATAPI 7 (and ver. 8). Download the spec. for free yourselves and try to stay awake reading them. You need substantial changes to the OS kernel and one or two levels up in the OS and the apps to support them. I left just before the MRDVRs became popular, but at the time I'm pretty sure no DVR was using those streaming extensions. We weren't getting any calls about those extensions, which was odd because we were pretty sure they didn't work that well and if anyone DID try to use them, they would have had problems and called us. But that might have changed since I left.
Besides, the Tivo kernel and its apps are available for download as well. You should be able to see if they are using those streaming extensions or not. I'll see if I can do that in my spare time, though I can't promise anything.
Regarding the whole "don't report errors" feature, we considered it very dangerous and totally useless. It gave too much control over our customers, who didn't know how hard drives worked. If an OS were truly tailored to an embedded DVR app, then it shouldn't freak out when an error is reported. What it should do is warn the user the hard drive is about to die. Errors occur all the time and are almost always corrected, but if the HD has trouble correcting the error that sector is normally moved from the bad spot to a good spot and not reported as bad to the OS. The hard drive only reports an error when it CAN"T correct the error or move the sector, which means the hard drive is circling the drain. I always thought DVR mfrs would use this feature to cover up defective hard drives and lower the amount of returns they would have.
Finally, I pulled the critical performance specs. from a WD Green drive and a WD AV-GP drive. This doesn't mean I used to work *there*, I chose these because they are used by a lot of people. I could also be comparing two generations of hard drives.
WD20EURS (AV) vs. WD20EZRX (Green)
Max. sustained transfer rate: 130 MB/s vs. 147 MB/s. The Green is FASTER. Note that one MPEG4 1080p stream only needs 22 Mbps, and one MPEG4 720p stream needs just 12 Mbps. So both are plenty fast enough.
Both have 64 MB cache.
Neither published their RPM speeds. Dead giveaway they are lower than 6000 RPM.
Max. power: 4.4 Watts vs. 4.1 Watts. The Green is actually a little cooler. It may be spinning a little slower.
Noise: 24 dBa vs. 27 dBa. The AV drive is a little quieter, though at those levels you won't hear anything more than 3 feet away.
Warranty: 3 years vs. 2 years.
Price: $99 vs. $96.
For the extra $3 and an additional one year warranty, I'd currently recommend the WD20EURS drive over the WD20EZRX for DVR and PC use. But I'd base my decision solely on those two factors. I also think the Green drive is a newer design than the AV drive -- it consumes less power AND it's faster.