Every x-acto knife, scalpel, chisel comes to a point where it needs to be re-sharpened.
Every HDD eventually fails.
Every angle grinder disk will eventually get too small to actually grind anything.
With all these three simple assumptions verified in the same day and quite a bit of frustration, I began this project.
To turn that junk that was piling up before me in something useful: a small emery wheel of sorts.
BEWARE: angle grinder discs are not meant for sharpening, they are meant for cutting: every disk shows a warning that tells you to make contact with objects only with the side of the disk; touching the surface of the disk with an iron bar will result in it shattering and splinters wil fly around and you'll get hurt.
However, an angle grinder packs much more power than an HDD elecrtic motor. With this setup if you push to much into the disk, it'll just stop spinning.
DISCLAIMER: as usual, if you hurt yourself following these instructions, that's because you didn't know what you were doing, and not my responsibility. Use gloves, eye protection, body armor, and if you're not sure how to do something, just don't. Don't risk it, and don't blame me if something goes wrong.