I worked on this WD20EARS-00MVWB0 drive, Sadle G6 family, with faulty head 0.
I was able to clone surface on heads 1-2-3-6-7 under MRT DE, i turned off head 0 in DE to be able to clone on those other surfaces.
Even if head 0 is totally unable to read its surface in the UA, it's still able to read SA but with some troubles.
Some modules were read slowly, some other were near to impossible to read. Module 04 was one of those near to impossible to read, i backup it partially by reading the relative track but i get corrupted module.

Mod 04 is not important for accessing UA, but i'm interested to find out a way how to rebuild it
I don't know if pc3k/sediv/wd marvel are able to rebuild it automatically, what i'm sure about is that MRT can't, so i find out how to rebuild it manually.

What was interesting in my case, was that mod 04 was unreadable also on head 1, so i rebuilded it manually and then i wrote it to both surfaces.
Now i can access and read this mod on both surface perfectly without any slow issue, this means that head 0 is still alive for writing too (at least on SA).

If you want to rebuild mod 04 on Sale G6 drives, do the following:
1) Download mod 04 from a fw resource which has same rom microcode of your patient drive. Most part of 04 module should be identical to your patient 04 module.
The only patterns that will be different are those from offset 0x100 to 0x117 and offset 0x0C (if your donor mod has same microcode, same head map, same head number, and most important same servo F/W version).
See pics:


2) Open with hex editor mod 03 of your patient drive, copy offsets between 0xBCF0 - 0XBD07 and past by overwritting donor 04 mod from 0x100 to 0x117

3) Now your 04 mod is ready to be written on your patient drive, you'll see offset 0x0C automatically corrected by MRT (or pc3000) at next access.

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My firmware database:
https://mega.nz/folder/O01DkBRI#MxP2J6ZNqXDcrX40I8MoQQ

Thanks for sharing helpful info

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A Recovery Service In Turkey . Veri Kurtarma Türkiye https://www.digitalverikurtarma.com
Donor Drives hdddonormarket.com

These are the capacity and TPI setings for all heads. This kind of data is located at the beggining of mod 04 and at the end of mod 3 of each WD drive....

Thanks for this details BGman

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My firmware database:
https://mega.nz/folder/O01DkBRI#MxP2J6ZNqXDcrX40I8MoQQ

This exercise raises an interesting question -- is it possible to rebuild critical modules from the data in other modules (eg selfscan logs)?

Is module 03h a critical module? If so, where would one find the data to repair it?

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A backup a day keeps DR away.

ARCO tests Logs and mod 04.

Is this feature present in any DR tool, or would one need to do this manually?

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A backup a day keeps DR away.

03=Current format
04=Format DCM 1
DCM = Disk Configuration Matrix
ARCO tests choose the best configuration from DCM and create mod 03

You could find out. ;-)

WDMarvel must be using one or modules in the SA for the regeneration. You could try deleting or renaming selected SA modules from your resource dump until WDMarvel complains that it can't find them.

You could start by deleting module 04 and observing WDMarvel's response.

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A backup a day keeps DR away.

Some new drives like FLITE, HubbleLite, etc.... don't have mod 04, so you can try on such a drive....
It's known that one cannot cut head on such drives just because mod 04 is missing and therefore mod 03 cannot be created.