Since the 1903 Update is still unavailable I contacted Microsoft and established a Remote Control session.
The technician attempted a manual Install and Update but encountered the same problem as myself. However, he identified the possible cause as:
https://docs.microsoft.com/en-us/windows/release-information/status-windows-10-1903?ranMID=24542&ranEAID=kXQk6*ivFEQ&ranSiteID=kXQk6.ivFEQ-EBUkbBN0tQPIko375G7amw&epi=kXQk6.ivFEQ-EBUkbBN0tQPIko375G7amw&irgwc=1&OCID=AID681541_aff_7593_1243925&tduid=(ir__lnkv1sorkskfrlt1kk0sohz30m2xj3jlshhwmixg00)(7593)(1243925)(kXQk6.ivFEQ-EBUkbBN0tQPIko375G7amw)()&irclickid=_lnkv1sorkskfrlt1kk0sohz30m2xj3jlshhwmixg00#490msgdesc
Error attempting to update with external USB device or memory card attached
If you have an external USB device or SD memory card attached when installing Windows 10, version 1903, you may get an error message stating “This PC can’t be upgraded to Windows 10.” This is caused by inappropriate drive reassignment during installation.
Sample scenario: An update to Windows 10, version 1903 is attempted on a computer that has a thumb drive inserted into its USB port. Before the update, the thumb drive is mounted in the system as drive G based on the existing drive configuration. After the feature update is installed; however, the device is reassigned a different drive letter (e.g., drive H).
Note The drive reassignment is not limited to removable drives. Internal hard drives may also be affected.
To safeguard your update experience, we have applied a hold on devices with an external USB device or SD memory card attached from being offered Windows 10, version 1903 until this issue is resolved.
Affected platforms:
Client: Windows 10, version 1903
Workaround: This issue has been partially resolved but to ensure seamless update experience, the safeguard hold is still in place. In the short term, we recommend you do the following workaround to update to Windows 10, version 1903. Remove all external media, such as USB devices and SD cards, from your computer and restart installation of the Windows 10, version 1903 feature update. The update should then proceed normally.
Note If you need to keep your external device, SD memory card, or other devices attached to your computer while updating, we recommend that you do not attempt to manually update to Windows 10, version 1903 using the Update now button or the Media Creation Tool until this issue has been resolved.
Next steps: We are working on a resolution and will provide an update in an upcoming release.
ETA Tried the workaround - it didn’t work …