Good work in this topic -- thanks to all. It is what I suspected. There are different model numbers for various reasons. Parts become obsolete or cheaper parts become available. Some buyers want different defaults programmed into the drive. Some companies put the lot number right into the end of the model number. Let's face it, you can't order 5,000 of something every month for 24 months and expect all 120,000 parts to be exactly the same. Especially when you have two or three manufacturers doing it for you. But the big difference here is in AAM. People should really have this set to as quiet as possible for a DVR. This is the lowest number possible. It's usually 128. All it does is slow down the head which increases seek time. Personally, my Samsung is so quiet in my Tivo you cannot ever hear it. The Western Digital OEM drive was way noisier in comparison.