Dear Graham,I suggest that you try to redo the procedure on another USB port, if it doesn't solve it, redo the procedure again with Media Creation Tools, download the Windows iso again and create a USB flash drive, according to the procedures below:
Download the tool according to the installed Windows 10 architecture. On here: https://go.microsoft.com/fwlink/?LinkId=691209
Run the downloaded setup and click Create installation media for another computer;
Select the Language, then select the edition and architecture according to the installed Windows 10;
Connect the Pen Drive;
Click Next;
Select the USB Flash Drive option and click Next;
Click Finish;
Boot with the Pen Drive; If you don't know how to access here: https://support.microsoft.com/en-us/windows/cri...
After creating a new flash drive, try this repair procedure with the media again.
I hope I helped, even more!