Based on my limited experience with opening hard drives (admittedly, nothing recent, but I doubt things have changed much), there are two styles of magnets (discounting those in the motor, of course) in hard drives:
One is a semi-circular "D" shaped magnet below the head positioning actuator coil; depending on the age of the drive, there may be one or two present (if two, one will be above, the other below), each mounted on a metal bracket with some kind of epoxy/super-glue or something, and the brackets are spaced from each other (and the coil) by either metal/plastic separators or molded in tabs on the metal brackets (part of the stamping process, I imagine); sometimes it is a combination. You'll usually find two magnets on older drives from the 1990s or so; I believe they used alnico or some other magnet technology, and thus needed to double up on them to make the magnetic gradient stronger (?). Later drives used rare-earth, which are much stronger (but more fragile). The magnets are essentially bar magnets with a north and south "end"; the coil changes it's magnetic field orientation amount to move the actuator along this gradient.
These magnets are fairly large, but that might be ok on a large perimeter; the only issue would be how to mount them to keep them from being hit by blades or dislodged in some other manner. Probably the best way would be to use a small trencher and bury them down 6-8 inches under the surface. There's also the issue of removing them from the metal brackets; you can use a hammer and screwdriver (or cold chisel) to do this, but you'll break as many as you release, generally. For my needs, this was too much effort for too little gain (might as well bury a wire). Which is why I'd rather purchase my magnets and golf-tees for the purpose.
The other type of magnet, which isn't in all hard drives, btw, is a small little "keeper" magnet (the thing is tiny, maybe 1-2mm in diameter?, and nearly impossible to remove from the plastic piece it is embedded in). It's purpose (I think?) is to, as part of some kind of electromagnetic assembly (?), hold the drive's actuator arm away from the platters on power-down. The arm on power-down quickly swings over to it's "parked" location, and this device latches it in some manner, so that the arm can't move when it is transported or otherwise handled. Generally they are not worth keeping when disassembling a drive, as removing them from the plastic bit can either be fairly easy, or nearly impossible, depending on the drive.
I learned all of this while, for one of my previous employers, disassembling a bunch of old drives for "drive destruction" (which was part of his contract with his clients); I gave him all of the metal platters and spacer rings (he wanted to make some wind chimes or something), I kept the motors, magnets, drive cases, and miscellaneous screws. I have a whole box of this junk. The drive cases, one day I intend to take to be recycled for the aluminum (and I should do it soon, as who knows when or if the bottom will fall on prices for the metal; though for now all it does is seem to rise). I won't get rich, but I might get enough money for a tank of gas or something.