Works for me. I used a Samsung 940X laptop and a Sandisk 80GB USB3
This is a UEFI machine, and I installed using manual partitioning to
re-use partitions on the external device, which contained an old Fedora
23 system. My purpose here was to preserve an encrypted /home partition.
/boot, /boot/efi, /root and a swap partition were reformatted by the
installer for the F25 system.
In order to boot from the external drive, I must use the EFI boot loader
- press F10 during start to display a list of boot devices, then select
the SanDisk USB drive.
The internal drive for this machine dual boots Fedora 24 and Windows.
Only the external drive was used by Anaconda for the F25 installation.
It might be possible to update GRUB on the internal drive so it can boot
F25 from the external drive, but I rather like the present state where no
change was made to the internal storage medium.