A:
Your Mac natively reads and writes to a format called "Journaling"
PC natively reads and writes to a format call "NTFS"
I think that drive is natively NTFS so your mac will not recognize it.
CAUTION:
There is a format they both read and write. BUT if you format your drive to that, NOW, you will lose what ever data is on the drive NOW. So if you go forward and want to make a drive they both can operate on (BACK UP YOUR DATA FIRST) on ANOTHER DRIVE then reformat your new one to a FAT32 format or EXFAT. They both will read and write to these. Link below to understand differences in FAT32 vs EXFAT
http://www.differencebetween.net/technology/software-technology/difference-between-exfat-and-fat32/