Unable to install Windows 8.1 using Bootcamp, system volume doesn't contain required free space?
To free up space in your EFI partition, you would have to first mount it. This can be done using the command given below. Enter the command in a Terminal application windows.

diskutil  mount  /dev/disk0s1


Below is an image of the entire contents of my EFI partition. I assume you can delete any folder and/or file other than what is shown below. (Actually, I am not even sure the Firmware.scap file is needed. I do not know what it is for.)

Click on image for a better view.


When finished, you may need to empty the trash. Finally, enter the command below to unmount the EFI partition.

diskutuil  unmount  /dev/disk0s1


If you are doing BIOS/MBR install, then what is posted below applies

From reading other posts, I thought 2015 Macs can no longer be installed in BIOS/MBR mode. So this would apply to older Macs.

To change the system partition to partition 4, you need to run the following commands in a Terminal Application window. You need to do this before attempting to install Windows.

INPUT=$(printf  "f  4\nw\ny\nq")
sudo  fdisk  -e  /dev/disk0  &lt;&lt;&lt;"$INPUT"


This will require your login password. This is normal.