Moving files from MAC

My WD portable drive is working fine with my Dell laptop but I’m having a problem when using it on my Mac.

There’s no problem when transfering files from the WD to the Mac but it won’t let me move a file from Mac to WD.

I’m sure it’s something simple, I just can’t figure it out. Any suggestions? Thank you

I am having the same issue. Works perfectly on my toshiba but on my mac - no dice on transferring from Mac to WD. Did you solve this issue? Please post up if you did. Any help from WD themselves??