This semi-frequently-updated site shows which hard drive models currently on the market (or discontinued) use what platter density, how many platters and how many read/write heads.
These attributes are important for several reasons: larger platter density generally equals higher data throughput performance, but may result in slower seek performance. Meanwhile, less disks and actuator arms mean less moving parts in the drive to eventually break, and can also lower the heat and noise output of a drive. I research and collect data from online, as well as from drives that I manage to test in real life, and put it all in these lists for you folks to look at.
Anyway, choose which manufacturer made your drive:
Note: I try to keep the information updated and accurate, but I don't always get things right. Thenceforth comes a disclaimer: use of this database is at your own risk, and all information in this database should be taken with a pinch of salt. For best results (and minimal heartache), please consult multiple sources other than just this site when searching for hard disk platter-related information.
A lot of the data on this site comes out of logical guessing; in other words, working out which platter density a given generation of drives uses by, for example, analyzing benchmark results; and, with that, using simple math to figure out the platter/head counts for specific drive capacities (rather than purchasing a bunch of drives and cracking them open - that's expensive!). These lists were originally created because most of the drive makers don't release this info to the public anymore. (Some older company datasheets - such as those for Seagate's Momentus 5400.2 notebook drives - do list platter/head counts for each capacity. In those cases, I've pulled their information into the lists for convenience.)
That said, if you see any drives/models that aren't listed or you feel are listed incorrectly, feel free to sound off in either the comments form of the appropriate section or through the Contact form, and I'll see what I can do.
Note 2: The new helium-based drives that some of the manufacturers (namely Hitachi/HGST and Western Digital) have released all employ seven or more platters and fourteen or more read/write heads. Enabling more platters and read/write heads than usual to be implemented is one of the main points of using helium, after all (six platters and twelve heads seems to be the limit for "fresh air-breathing" drives, as of Q2 2016). For that reason, along with a lack of time on my part, I will not be adding these helium-based HDDs to the lists, at least for the time being.
Update list (major updates only):
2016/06/15 --- [2.5"] Added Samsung SpinPoint M9TU series.
2016/06/13 --- [2.5"] Added Samsung SpinPoint D8X series.