microsoft url is
https://docs.microsoft.com/en-us/windows/release-information/status-windows-10-1903#532msgdesc

scroll down to Error attempting to update with external USB device or memory card attached

it says
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.

my windows update simple says 'your computer is not quite ready yet' which i assumed was Microsoft waiting for a fix