install F25 Beta in legacy mode on an 64GB flash medium. And surprise
it
works.
I think this suggests there is some configuration or BIOS problem with
EFI and USB in your machine. Both Chris Murphy and I performed
successful EFI installations, he with Apple and I with Samsung hardware.
It may be impossible to accurately gauge the prevalence of this problem
with the limited hardware coverage available for pre-release tests. On
the positive side, you have discovered a way to avoid the issue that may
help other users who experience similar difficulty.