lonelydad wrote:
I'll admit I didn't phrase things very well, but there are two distinct
actions going on here.

1. Since Microsoft has seen fit to not yet offer to upgrade my system to
1903, for no reason that I can discern, I am seriously thinking about
upgrading it myself, thus the downloading of the current 1903 version.

2. I just acquired a large USB drive, and want to make it bootable, for
reasons not really connected with #1. Thus the thought to use the Microsoft
manual download process to make the USB drive bootable, rather than just
download the ISO.

My understanding is that if I run startup from the USB drive WITHOUT
booting from it, in other words from a running 1809 Windows system, that I
will be able to perform an upgrade, and not a clean install, which would
happen if I booted from the USB first before running startup from it.

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.

*******

If you wanted to do a Clean Install, you could have
MediaCreationTool1903 make your 512GB USB drive into
a "bootable USB stick". That's a possibility.

*******

You can also use Macrium, to turn the 512GB drive into
a bootable USB device. That would be WinPE based.

You can also put a backup of C: onto the same partition
as the Macrium component parts. This turns the 512GB drive
into both a boot source, as well as a data source for
doing a restore of C: .

Any more complicated setups are overkill.

Paul


Paul