Windows 10: latest Win10 boot medium wont boot

Version 1703 is not available via the media creation tool until at least April 5:
Managing Windows 10 Creators Update rollout for a seamless experience - - Windows 10 Forums

Also, you can right click on the .exe file for the Media Creation Tool and select properties, then details:





As long as it says product version 14393, it's going to create a version 1607 USB flash drive. The product version will change to 15063 when it is updated to create version 1703. Media Creation Tools will only download the version of Windows 10 that was available when the tool itself was downloaded. You have to download a new Media Creation Tool each time it is updated and a quick check is to look at the detailed properties of the mediacreationtool.exe file.

As long as the major build number (14393 current, 15063 likely for version 1703) is the same, you are always just 1 cumulative update away from being current.

 

Thank you for that very detailed information - I'm happy to wait till April 5 or later before having another attempt - at least now I know the criterion on which to base my judgement about the likely Winver. And I also know that I can clear away the earlier downloads. Is DiskCleaner the best way to get rid of the $Windows folders and the other associated folders ?

 

I prefer option two here :
Open and Use Disk Cleanup in Windows 10 - Windows 10 Performance Maintenance Tutorials