Accessing PC Backup on My Passport with MacBook Pro

Hello. This is my first post, so pardon the newbie question.

I have a WD MyPassport Ultra 1T external drive that I used to perform routine backups on my old desktop PC. That old PC has finally died and I’m replacing it with a MacBook Pro. 

I am able to read the files on the MyPassport, but cannot write to the disk or update the files. Specifically, I’m using the iTunes backup on the external drive to setup iTunes on my Mac. iTunes (the Mac) is able to read the files and all works well. Until I need to change one of the files or write new files to the iTunes library on the disk.

Seems to act as if the Mac does not have access writes. Since my PC is dead, how can I authorize the MyPassport drive to allow write access for my Mac?

Thanks in advance for the help.

I don’t know much about Macs but check this post and see if it applys to your version of Mac.