Media Creation Tool Error 0x80042405 0xA001B

OK, I see your numbers. Too, too bad I hastily deleted mine.

(1) Ah, right, it was $Windows.~BT inside Windows.old (not the other way around I said).

(2) Ah, that's right, "Previous Windows installation(s)" (not what I said).

(3) I see yours is a Legacy install:     System Reserved, Windows (C:), Recovery;

    not the UEFI install that I've got:  EFI, MSR, Windows (C:), Recovery.

Here are my thoughts...

(a) Maybe MSFT continues to do it the old-fashioned way for Legacy installations.

(b) Maybe my eye cataracts have blocked out most of my Windows.old.

Anyway - I'll make pictures after the next feature update (new Windows version) & post them into this thread. I'll be flabbergasted, if it looks like yours.

Edit: And here are my partitions. The upgrade didn't add the Recovery (WinRE 1903), because it was already there & large enough. But it did update it, because Shift+Restart gets to a 175 Command Prompt - just as you promised in that other thread. 

Note: Disk Management doesn't show the 16 MB MSR partition. That's normal.

PCR, 

As for your # 3) ,

I have 5 systems 3 are legacy and 2 are UEFI, the logic is the same,

Windows.old was over 23GB on my UEFi machines too.

The only difference on the UEFI it did not need to add a recovery partition because the existing one was big enough. It may add an extra one or expand the existing one if there is not enough space.

Also Disk Management does not show all the partitions, there is a 16MB of type other that you don't see on disk management, but you will see it with diskpart  -> list partition.

See below.

DISKPART> list disk

  Disk ###  Status         Size     Free     Dyn  Gpt

  --------  -------------  -------  -------  ---  ---

  Disk 0    Online          476 GB      0 B        *

  Disk 1    Online         1863 GB      0 B

  Disk 2    No Media           0 B      0 B

  Disk 3    No Media           0 B      0 B

  Disk 4    No Media           0 B      0 B

  Disk 5    No Media           0 B      0 B

DISKPART> select disk 0

Disk 0 is now the selected disk.

DISKPART> list partition

  Partition ###  Type              Size     Offset

  -------------  ----------------  -------  -------

  Partition 1    Recovery           499 MB  1024 KB

  Partition 2    System             100 MB   500 MB

  Partition 3    Reserved            16 MB   600 MB

  Partition 4    Primary            476 GB   616 MB

OK. I await the next upgrade, & chances are good I'll be flabbergasted to see it gives me a full sized Windows.old. Did you actually see your user folders (Documents, Downloads, Pictures, etc.) inside your Windows.old? I definitively recall mine were absent.

That's right, the MSR (Microsoft System Reserved) shows up in DiskPart & in 3rd-party partitioning tools like AOMEI Partition Assistant...

That MSR is a strange one. It doesn't show up as a separate entity in a Windows system image backup, either. But I discovered it is within the .vhdx of the Windows partition. But I can't recall how I made the discovery or how to repeat it.

PCR, 

If you go in the  Windows.old 'Users' folder you should find all that stuff  needed for the various users to go back.

The files may not be in the windows.old folders if they were already moved in the new folders, they save the structure and the folders, they don't need to save the same stuff twice as long as it is available on the C: volume one place or the other, they can go back to the previous version and move the files where they belong, the option to go back is only good for 10 days anyway... 

Here is the same disk cleanup on a UEFI system, you can see 25.3GB for the previous windows Installation