Airport Express: External Hard Drive Setup - Access with Mac AND PC


Good evening -
As a follow up to my prior post - I was hoping to get some additional information regarding attaching an External Drive to my Airport Extreme and getting it to be accessible by both a mac and PC.
I have attached a G-Drive Mobile formatted for Mac to my Extreme Airport; when I do this I can see and access it via my Mac but I cannot see it on my PC.
Does anyone know why this might be? Do I need to partition and format the drive for both or is it something simpler than this?

Thank you for your help.