Hello,
If you where using this drive on a Windows and want to use it on a Mac now you will have to format it in order to be able to write to it. However, If the drive is not seen when you connect it to your computer I recommend that you try some basic troubleshooting:
1- Connect the power adapter to the wall directly.
2- If the computer is a desktop, connect it on the back USB port (directly to the PC, not through a hub).
3- Go to disk utility, and verify if the drive shows.
-If it shows in disk utility then click on the link below to get information how to format it. (note that formatting the drive will delete all data stored on it)
How to partition and format a WD drive on Windows (8, 7, Vista, XP) and Mac OSX
