This periodically-updated site attempts to keep a record of which hard drive models, both current and discontinued, found in desktops and notebooks, DVRs and cars, but generally with a spindle speed not above 7200 RPM, utilize what capacity of platters, how many platters, and how many read/write heads.
These attributes can be important for several reasons: greater platter density generally allows for higher data throughput performance, but may result in slower seek performance. Meanwhile, fewer disks and actuator arms mean fewer moving parts in the drive to eventually break down, and can decrease 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 arrange 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. So, here's a disclaimer: the use of this database is at your own risk, and all information in this database should be taken with a pinch of salt. Also, the contents of this database may change at any moment! 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 from "logical guessing": working out which platter density a given family of drives uses by, for example, analyzing online benchmark results, and then using simple math to figure out the platter/head counts for each individual model. This is done instead of purchasing a bunch of drives and cracking them open -- by far the most effective way, but also the most expensive and wasteful. These lists were originally created because most of the drive makers had stopped releasing such information to the public. (Some company datasheets -- such as this one for Seagate's Momentus 5400.2 -- do list platter/head counts for each capacity, or at least provide enough related specifications to get there manually. In those cases, I've pulled and/or adapted their information into the lists for convenience.)
In recent years (mainly since ~2009, by my estimation), individual models with multiple possible configurations of platter density, platters, and/or heads have been reaching market, with the only sensible ways to tell the difference being weighing and benching (each with the inconvenient prerequisite of having the individual(s) in question on hand). The manufacturers' reasons for doing this can only be guessed, and not every model is necessarily affected, but the likelihood of a particular model to be sold with such an "alternative" configuration may vary depending on the time it's spent on the market, among other possible variables. Nonetheless, the head/platter counts for newer (from 500GB/platter for 3.5"; 250GB/platter for 2.5") units on the pages, especially, should be considered only as the best-case scenario for those units. Models which received increased scrutiny for this problem will be accompanied in the pages with way(s) to tell their variants apart, while refurbished, recertified, or repaired units of any build date (due to the nature of the makers' restoration process in many cases) should be considered as a lost cause.
That said, if you see any models that aren't listed or that you feel are listed incorrectly, feel free to sound off in either the comments section of the appropriate page or through the Contact form, and I'll see what I can do. If possible, please provide a screenshot of benchmark results from HD Tune or CrystalDiskMark (or equivalent), which displays maximum sustained read speeds performed by a blank drive; mentioning the physical weight (use a scale) of the HDD(s) in question is also highly encouraged.
Note 2: The new helium-based HDDs basically all employ seven or more platters and fourteen or more read/write heads. The main point of incorporating helium is to enable such huge amounts of platters and heads to be used, in order to get around the apparent limits (six platters and twelve heads, as of Q2 2016) of "fresh air-breathing" drives. For that reason, I will not be adding these helium-based HDDs to the lists.
Note 3: For a while between Q4 2017 and Q1 2018, this site was set to "private mode" in Blogger's settings, thereby completely cutting off public access. This was because I was planning to move the site to a more robust and better-organized content system. That fell through, so the site is now back to "public mode", as it was before. Apologies from yours truly for any inconvenience this may have caused.