Hoo boy, this is a whole thing.. I have not seen evidence that there has been a 3rd party service industry for hard disk repair after the era of mainframes and minicomputers... they had big old washing machine or PC case sized hard drives that might have had tech service possible. Since we've had the 5.25" and down sizes though they have been regarded more as consumables. The only people much interested have been the data recovery specialists, and they aren't into making one drive be as good as it can be, they are into creating optimum conditions to extract data from platters, which may consume the original drive, plus half a dozen identical ones, if the customer is paying a lot, the surface is breaking up and they need to keep replacing heads. Basically, they are part swappers rather than repairers. Then also it's easier to get into the Freemasons than HDD data recovery, like a real secret society it is.

Anyway, so I don't think service manuals exist outside of folders of original tech data at manufacturers warranty departments, who may have tossed a lot of it when the last sold drives were 5 years old.

I'll give you a clue about the origins of IDE though, there were hard drives, and there were controllers, and at that time it was one controller for 2 or maybe more hard drives. The controller was plugged into an ISA slot, the AT bus. Then the controllers were Integrated into the Drive Electronics, leaving an ATA interface between the computer and the controller... with me? Apart from some signals to argue about which of two controllers could use the bus and other control and indication, it's mostly AT bus... at least that's where it began.

Agree you will probably not find an exact schematic for a specific IDE model, those would have been kept in the engineering offices (in secret to avoid others from copying) where they would have then created a PCB layout and parts list sent to the factory where the drives were made. With so many manufactures and models there would have been possibly 100's of schematics over time but due to copyrights/secrecy would never have been released publicly....
The SCSI BUS and IDE Interface book from ~1997 has some basic IDE timing/register/etc info: https://vtda.org/books/Computing/Hardware/SCS … face_2nd_Ed.pdf

For HDDs you got russian ACELab PC-3000 .. and pretty much nothing else. They are around since mid nineties, provide hardware an extensive database of hardware nd firmware info. Wouldnt try buying their wares in current climate, but you can find old pirated versions from ~2002 (v14) on the net if you are curious in how all of this works.

Fixing drives nowadays is 99% to recover data, last 1% is scammers trying to pass defective/overworked drives are refurbs with cleared SMART.

thank you guys for your insightful feedback