Welcome to the community, Dani31!
I'm sorry to hear about your issues with the WD My Book drive.
Since the model is specifically for Mac, it means that the file system is HFS+ which is supposed to work with Mac OS-based systems. In order to be able to read/write from both the Windows computer and the Mac computer, you'd need to reformat the drive in FAT32. However, this file system comes with some limitations which you can read about here:
http://products.wdc.com/support/kb.ashx?id=DtMWim
Can you please post a screenshot
(using imgur perhaps) of Disk Management on your Windows 8.1 PC?
Unfortunately, if the drive has shown up as 'Unallocated' and you've formatted it there, that means that your ITunes library might have been erased as well.
I'd recommend you to back up any data from the WD My Book that you are able to access and re-format it for use in both operating systems (Mac OS X and Windows OS). Here's a tutorial from our KB that explains how to do this:
http://products.wdc.com/support/kb.ashx?id=kryJ0O
Keep me posted though! I hope this helps!
SuperSoph_WD