This is my setup:
1. Mac Mini Late 2014 with 2TB Fusion Drive.
2. Windows 8.1 Pro x64 full version ISO downloaded from Microsoft Store.
3. Sandisk Extreme Flash Drive (I have tried both 16GB and 64GB).
4. Bootcamp partition = 500GB.
5. Other USB devices plugged in: Corded USB Mouse and Apple Extended Keyboard (both of which are required to enter the serial number and navigate the installer window).
6. No additional storage devices of any kind are connected aside from the internal Fusion drive and the installer drive.
7. Booting from the EFI portion of the Bootcamp created Windows installer.
I have read that I should try using a USB 2 flash drive rather than a USB 3 flash drive, so I will try that next. I fail to understand how this could make any difference.
If you have any other tips, please let me know.
I have installed or helped other people install bootcamp many times over the course of the past 2 years. Every single time it has caused endless headaches and literally weeks of wasted time.
I have read endless posts on what is causing these problems. I will consider summarising these for anyone else having the same problems. Everyone has their theory so far about this Bootcamp nightmare, but so far there doesn't appear to be any one magic solution.
Apple's inability to address the issue suggests that the problem may be with the Windows installer. Perhaps Microsoft is deliberately causing this?
I suspect the two most likely causes of these issues are the downloaded ISO or the Thumb Drive.
If you have any better idea, please let me know.
On another topic, how to I rename the NTFS partition back to BOOTCAMP? All options to rename this partition are greyed out in Disk Utility...