Hy
Had a problem with my 1TB ST31000528AS with the latest firmware CC49
After a power problem it wasn't recognised corectly ... had 32MB RAW
Idiot me went to rewriting the firmware manually ... but instead of writing PHCC494H.LOD as I was supposed to, I wrote PHCC491H.LOD
Now the damn HDD keeps trying to spinup
Even tried with a USB-RS232-TTL phone data cable modified to try the BSY error repair and rewrite the correct firmware to the HDD... I get errors
What can I do now? Can I use the data cable to issue some commands for the drive to be recognized in BIOS and retry a firmware flash with the correct firmware?

Where did the idea of using a definite file instead of using the package provided by Seagate came from ?

Well .... actually from the fact that force flashing with the right file did nothing to the drive
I now realize that flashing was not the solution... but the TTL cable was the way to go... since it was a 0B LBA after all in BIOS even if in windows the drive was detected as 32MB RAW

Had a feeling... Anyway , there was a problem underlying so it's difficult to tell. At present I think there is no or little chance to fix the problem without dedicated equipment, let alone data that is maybe compromised. Everything depends on the amount and position of firmware data that has changed.

Any chance I could write the firmware one byte at a time from the Terminal?.... I'm willing to do that if I do not find a local service with dedicated equipment...

In simple words NO you can't.

@op
You did not mention: you need data recovery OR drive being "fixed"?

Depending on what exactly it is, some things could be done. Needless to say, not on your own. Maybe by pro.

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Hard Disk Drive (HDD), Solid State Drive (SSD, SATA, NVMe, etc), USB Flash Drive and RAID Data Recovery Specialist in Massachusetts

I'm trying to repair it and restore it as it was
If I find a working HDD with the exact specs and hot swap the board from the working HDD to my HDD (with /2 Z and U after hot swap) in MS DOS and after the hotswap I try force rewriting the firmware.. the correct one this time... will it work?
Or will the U command make the HDD try to spinup again and again?

No it won't work. Part (very small part) of the fw is on ROM, THE OTHER IS ON THE PLATTERS. The disk itself won't accept anything in this state. Maybe some "trick" works, but I am not aware of "end user tricks" , we use "pro tricks" with dedicated equipment and even then some problem comes out, go figure....

Anyway, depending on what you do, maybe you can more or less and by kicking it hard restore the FW but it is yet to know if then the adaptives are still correct and translator works (in a few words : if the drive works).