Paul wrote:

Since you seem to want to do an Upgrade of your existing Win10 OS to
1903, just downloading the ISO and storing it on C: is sufficient.
You don't even need the 512GB drive to achieve your ends.

Upgrade Install:

1) MediaCreationTool1903.exe
2) Save as Windows.iso (on the C: drive if you want).
3) Right click Windows.iso, select "Mount" from the top of the menu.
4) Run "Setup.exe" from the resulting virtual DVD drive
that appears with a new drive letter.

As an aside (me interjecting an inquiry):

Since I'm considering the similar action (manual install of 1903), does
the update occur entirely within the current Windows session without
requiring a reboot that needs to reread the virtual drive? While the
shutdown and reboot will perform the file replacements, I'm wondering if
the reboot will be configured to resume any remaining setup.