The attached screenshot is from this MRT video which explains how to access a WD USB drive in SATA mode:

https://www.youtube.com/embed/kZK9ltcHWcs

The capacity reported via SATA (not USB) is 3906963632 sectors. This is 65536 sectors less than the IDEMA standard capacity, 3907029168. I have seen this behaviour in other threads. In those threads an examination of module 0x02 reveals that all 5 capacity parameters reflect the full IDEMA capacity, not the actual reported capacity. This same reported capacity is visible in the drive's Identify Device info sector when accessed via Linux, so it is not some artifact of PC3K or MRT.

The capacity reported via the native USB interface is even less by a few thousand sectors.

In another thread the difference was 65535 sectors rather than 65536.

What is going on? Where is this capacity value coming from?

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A backup a day keeps DR away.

I'm not entirely sure because I've only had this problem once and I haven't analyzed it that deeply, but maybe encryption and a boot partition for the WDunlocker application will be a clue?..

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Data recovery Poland (RAID, HDD, SSD) - https://hex-serwis.pl

Normally it is the USB bridge firmware that hides the "SmartWare" area from the OS, but when the bridge is removed, the full UA is exposed. However, in this case there are still some hidden sectors even after the drive is accessed directly via SATA and the bridge is eliminated. I'm wondering whether the drive knows that the end of the UA is reserved for an encryption key, and perhaps this restricted access is a recently introduced security feature. The bridge firmware would know how to access this area, possibly via a new VSC.

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A backup a day keeps DR away.

so i try to reply here.
Since i do not think i have any of these enclosures at hand, i cannot really test my suspects.
they are:
- the disk is SED, working transparent when not locked
- it has the smartware executable on the user partition, so user can start it and lock the drive with pwd
- SED uses ATA pwd to lock, it can be disabled but then data is crypted
- SED key is not stored in key blob at end of user area but in SA files (not 25 and 38) that's clear from the thread you linked: https://forum.hddguru.com/viewtopic.php?f=11&t=38894
- i share your suspect that if the drive is locked, the bridge presents a VCD partition which it sources from the drive in some way.

do we have an image of such VCD partition?

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Adatmentés - Data recovery

See this thread:

https://groups.google.com/g/datarecoverycertification/c/0XC2mZEB38M

The OP in that thread tried to access that hidden area via SATA in many ways, including with an unlocked PCB in Linux, but was unable to see it.

Allow me to repeat what I wrote elsewhere:

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A backup a day keeps DR away.