The problem is the fact that HDD motor have 4 pins rather than 3.
That's what the article explains. Because from the 3 signals you get 1 is always the negated signal of another pin, 2 are enough for this use case. So you have 3 signals and one ground makes a total of 4. Have you read the whole article or just the first step?
As you can see in the picture below the schematics two signals are the negated signal of each other. You can eliminate one of them and still have the information you need for a rotary encoder.
There are no silly questions if you don't know something. A negated signal is always the opposite of the other one, meaning when signal A is HIGH, the negated signal is LOW and when signal A is LOW, the negated signal is HIGH.
From the schematic : Which one is negated by which one ?
Unfortunately this doesn't get clear from the article. I would just try and attach a scope to two pins. The scope output will show you quickly which are the right ones. Probably this changes from one drive manufacturer to the other, so you have to check anyway.
pylon:
Unfortunately this doesn't get clear from the article. I would just try and attach a scope to two pins. The scope output will show you quickly which are the right ones. Probably this changes from one drive manufacturer to the other, so you have to check anyway.