No - it is not a problem of a slow USB drive!!!
For a 'flat-file' (not VHD) install:
I used a Lexar Jumpdrive which I can make a Removable drive or a 'Fixed-disk' drive by using the BootIt.exe utility.
So I am using exactly the same drive for each test.
If the Lexar Jumpdrive is set to be a Removable
USB using BootIt.exe then Win8.1
and Win 10 will not boot to Desktop (everlasting spinning circle of dots).
However, Win7 and Win8 do work OK.
If the Lexar Jumpdrive is set to be a 'Fixed-disk
' USB drive using BootIt.exe, then Win7/8/8.1/10 all work OK.
This is a known issue with Win8.1 and Win10
- will not boot from a Removable USB drive unless a VHD is used.
I have also tested with a very fast SanDisk Extreme Cruzer USB 3.0 64GB Removable USB drive - same problem. Many others have reported the same problem to me.
If installing with WinToUSB to a Removable
USB drive, you should not allow Win8.1 and Win10 to do a 'flat-file' install because it does not work - only allow VHD option.