Wd passport 1tb

WD External and Portable SSD Portable SSD
1

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 

2

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