If you are looking for an external hard drive to connect to a DVR, my advice is to buy one that is specifically designed for that purpose and it will work perfectly without having to worry about the exact specs. The Western Digital "My Book AV DVR Expander" is one example of such a product that has both an eSATA and USB interface, and you can easily find an online retailer with a web search. Other examples you can search for are the Iomega "DVR Expander Drive" and the Seagate "Showcase DVR Expander". Future Shop sells this Iomega model, which will work great with any DVR that has an eSATA external drive interface.
The specs you quoted (7200 RPM and 16MB buffer) probably came from the website of one of the cable TV service providers. Those sites usually also list another extremely important spec you didn't quote which is that the drive should never power down while idle - the drive must be powered up and running 24/7. Most of the cheap external hard drives sold for use with computers will fail to meet this requirement, while all external hard drives sold for use as a DVR expander are specifically designed to meet this requirement.
If you are into a "do it yourself" approach you could buy a bare hard drive that meets the specs discussed above (look for the "AV" versions of drives manufactured by Western Digital or Seagate) and install it into an external drive enclosure.
