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In the context of a failed hard disk (HDD) printed-circuit board (PCB). It is commonly asserted that swapping a failed PCB with an identical replacement (or donor) PCB will not work. The common reason given is that ROM contains unique "adaptives" which have written to the drive during its lifetime.

However, and maybe I'm misunderstanding this, but how does adaptive data get written to the ROM chip if its "Read-Only Memory"?

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    there aren't really a lot of ROMs these days. most of the time its EEPROMS. true ROMs are manufactured with their "code" already packaged in. Now-a-days though, manufacturing supply chains are much more complicated, so the flexibility of using an OEM-modifyable chip makes much more sense for everyone.
    – Frank Thomas
    Commented Feb 18, 2021 at 23:23
  • "Why does chip have to be transferred?" (the title) and "how does data get written?" (the body) are distinct questions.
    – Kamil Maciorowski
    Commented Feb 18, 2021 at 23:25
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    Technical terminology gets misused outside engineering, e.g. by marketing and hobbyists. E.G. smart phones are typically advertised with X amount of "ROM" and Y amount of RAM. The reference to "ROM" is invariably storage for firmware implemented with NAND flash, which of course is not RAM so it's called "ROM".
    – sawdust
    Commented Feb 18, 2021 at 23:53

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The ROM is, as commented, most likely an Electrically Erasable Programmable Read Only Memory (EEPROM), a type of memory that is generally only able to be written once before needing a special procedure to erase.

This EEPROM will be written at the factory at the time the PCB is married to the actual hard drive and likely contains the drive firmware and special calibration data required for the software to deal with the particular sensitivities of the drive head it is connected to.

Different firmwares may arrange things subtly differently on the disk, meaning that another board with different versions may not understand the underlying disk format.

Different calibration data may mean that whatever magnetic readings the drive head uses gives wrong results when passed through amplifier circuits.

Transferring the ROM over will transfer the correct firmware and/or calibration data needed to read the drive correctly.

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  • So this would mean that a donor disk's PCB, manufactured on the same day and from the same batch, would probably be compatible with the disk whose PCB is defective?
    – daikin
    Commented Feb 18, 2021 at 23:44
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    Maybe at best. Different drive heads may require different calibration, microscopic differences in thickness of material on the disk platter may need to be accounted for differently in the calibration. Platters and heads will not all be made at the same moment and even if they were may be in a different order in each drive which might have double sided platters (two different orientations) or multiple platters making more different combinations. I'd suspect that you would be lucky to find two drives made on the same day that actually have the same calibration data.
    – Mokubai ♦
    Commented Feb 19, 2021 at 0:06
  • Well, more common that you would think. Take for example, the typical off-the-shelf pre-rolled NAS device. They often contain HDDs from the same production batch.
    – daikin
    Commented Feb 19, 2021 at 0:09
  • Same batch of things put together, not necessarily the same batches of subcomponents and the subcomponents used are of a kind that can vary within batches, let alone across batches, depending on whether they were at the start or end of their own particular batches. Lots of room for differences in calibration of the final product.
    – Mokubai ♦
    Commented Feb 19, 2021 at 0:21

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