This 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 random seek performance, while less disks and actuator arms means less moving parts in the drive to eventually break, and can also lower the heat and noise output of the drive overall. I find and collect the data 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 my best 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 responsibility. Please consult multiple sources other than just this site when searching for hard disk platter-related information for best results. A lot of the data on this site comes out of logical guessing. After all, these lists were originally created because most of the hard drive makers don't tell the public about this stuff, so some (I guess you could say passive-aggressive) detective work is needed to figure out the head/platter counts - rather than purchasing a bunch of drives and cracking them open, as, first of all, opening a drive renders it unusable forever, and second of all, I am far from rich.
That said, if you see any drives/models that aren't listed or are listed incorrectly, feel free to let me know in either the comments of the appropriate section or through the Contact form, and I'll see what I can do.