I would use GParted to format the separate disk.
first you have to install GParted :
open a Terminal by pressing ctrl + alt + T
Then enter the following:
sudo apt-get install gparted and wait until the installation is complete. After that type in the terminal
sudo gparted and hit enter.
A window should open.
Follow the instructions in the link: Format a disk using GParted
- Open GParted in a terminal with
- In the upper right corner you can choose the HDD
- Choose the HDD where Windows 8 is installed on.
- Right click on the HDD (like in the foto) and choose
Format to and then the File System you want it formatted in. I would recomend you format it in NTFS.
- After that there will popup a field at the bottom saying Format "Path" as NTFS. Right click and press
Apply all Operations.
- Press Apply.
- Wait for the Operation to finish and your done. Your disk is formatted.