Wd passport 1tb

I’m trying to format my hard drive ,its primarily for my Mac but I have an old windows machine that’s got loads of vids on etc,

my problem is when I try to format using the wd instructions there is a message on the screen that says this partition cannot be modified .

Can any body point me in the right direction 

Or would it be easier to buy another hard drive for the windows machine then get software for Mac to read the windows ???