Assuming you have a complete, working donor drive, I would generate a patched donor ROM for the donor PCB on the donor SSHD and then initialise the NAND. I would then install the original or patched patient ROM on the initialised donor PCB and install it on the patient HDD.

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A backup a day keeps DR away.

I generated and installed the donor firmware patch on the donor PCB. I can access it with Ctrl+Z, but unfortunately, I get the same error, even with the HSA disconnected.

Request spin / disc table recovery

ASCII Diag mode

F3 T>/O

F3 O>I

Failed to load overlay 00000004

LED:000000BC FAddr:00004264

I bought the PCB on eBay, and it came with that firmware on the SPIFlash (HPM2), but the contents of the NAND flash memory are unknown.

Can anyone recommend a software tool to upload the diagnostic overlay?

Did you do this on the donor HDD? Do you have a complete donor HDD, or just the PCB?

_________________
A backup a day keeps DR away.

I'm not a pro, but this is what I think is supposed to happen.

The drive powers up with the patched ROM and provides basic access to the terminal. When you invoke the "O>I" command, the firmware attempts to retrieve firmware module 0x0004 from the reserved System Area (SA) in the discware on the platters. This module contains the code that implements the "I" command. The build version of this module must match the ROM version.

Unfortunately, the NAND flash memory not only caches user data, but it also caches those firmware modules. This means that your donor's NAND contains cached copies of the donor's 0x0004 module. When you install the patient's patched ROM on the donor PCB, and then start the patient HDD with this combination, you inherit the donor's 0x0004 module. Instead of loading this module from the SA, your patient loads it from the NAND cache, which then results in a firmware mismatch.

You need to clear everything in the donor's NAND, but you can only (?) do this on the donor drive.

Alternatively, it could be that the donor's NAND contains cached copies of the donor's ROM components, which would once again result in a firmware mismatch.

_________________
A backup a day keeps DR away.

I don't have a donor SSHD, only a donor PCB. I'm looking for a complete donor board and then I'll repeat the process. That will take me a few weeks. I'll update you as soon as I have any news.

Thanks to everyone for your help.