In the 1809 internal recovery environment...
(a) C: is put onto the defunct Win 8.1 recovery partition -- it should have no letter.
(b) H: is put onto the Windows partition -- we want C: instead.
(c) G: is put onto the 1809 recovery partition -- it should have no letter.
(d) D: remains on that HP partition (which likely is a Win 8.1 factory reinstall partition) -- OK.
In the 1909 external recovery environment...
(a) C: is put onto the defunct Win 8.1 recovery partition -- should really have none.
(b) The Windows partition gets no letter at all -- sheesh!
(c) The 1809 recovery partition gets no letter -- as it should have none.
(d) D: remains on that HP partition -- OK.
Let us fix it in the 1809 environment & see whether it has an effect on the 1909 environment. If not, we'll try to fix that too. If we get these letters straightened out, it will be worth trying the upgrade again. It may become less confused during its reboots.
I don't expect the letters we remove will remain that way. But I want C: to remain C:, & D: to remain D:.
So...
(1) Plug in your 1909 Installation USB drive.
(2) Shift+Restart for the 1809 recovery environment.
(3) "Troubleshoot > Advanced Options > Command Prompt"
(4) Enter...
DiskPart <<< Enters DiskPart
List Vol <<< This shows the volumes with their drive letters.
Does it look as above...?...
C: on Windows RE & it is volume 1
H: on Windows & it is volume 2
G: on the 492 MB partition & it is volume 3
If it looks like that, then....
Select Vol 1 <<< Select the Windows RE volume.
Remove Letter=C <<< Removes the letter "C".
List Vol <<< You should see "C" is gone. And let's not put another letter on it.
Select Vol 2 <<< Select the Windows volume.
Remove Letter=H <<< Removes the letter "H".
List Vol <<< You should see "H" is gone.
Assign Letter=C <<< Puts letter C onto Windows.
List Vol <<< You should see "C" is added to Windows.
Select Vol 3 <<< Select the 492 MB volume.
Remove Letter=G <<< Removes the letter "G". Let's not put another letter on it.
List Vol <<< You should see only C, D, E & F remain.
Exit <<< Exits DiskPart
(5) Exit the Command Prompt with its "X" or type "Exit".
(6) Click "Continue" to get back to Windows.
In Windows, Disk Management should look as always. C: & D: are where they belong.
Now -- as before -- go back in & show us a picture of DiskPart for both 1809 & 1909.