The behavior of this drive was confusing me so i took the pcb off to have a look. Don't you love it when customers don't tell you they have been experimenting?

Thats a 1N4007. I like to find the youtube videos that suggest these experiments and check with the customer if its the video they watched to do it...

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WTF WTF WTF!!

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That diode should have had no impact on the behaviour of the HDD. In fact Toshiba PCBs often [stupidly] use a Schottky rectifer in that position to protect against reverse voltages. Such diodes don't provide any overvoltage protection.

I would look elsewhere for the source of the "confusing" behaviour.

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maybe the diode is not there for reverse voltage protection. Im not going to pretend to understand circuit theory more than a basic level, but I have read some write-ups of circuits that use components to do things like reduce signal noise, or unconventional uses of common components that when explained is, or appears to be, clever. Is there any other possible reason for a schottky in that possition?

A reversed biased Schottky diode wouldn't do anything in that position. In fact a reverse biased 1n4001 GP rectifer is often used as a "dickhead diode" in automotive electronics.

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A backup a day keeps DR away.

he would like to find

"I like to find the youtube videos that suggest these experiments and check with the customer if its the video they watched to do it..."