Using a Windows External Drive Library the default library for a Macbook

I have a Windows PC that was running my iTunes library on an internal drive. The internal drive spat the dummy (kept crashing) so I now run it from an external drive. My problem is that I have a MacBook with a 128Gb SD card that is storing my Mac library. I want to use my MacBook pro for my "Master" library with the external drive but when I try and do that the Mac tries to create a new library and copy the contents of the drive onto itself.

I have 3 Terabytes of media and there is no room (it is a 4Tb drive) and the MacBook only has a 500Gb SSD.

Is there a simple way to tell iTunes that the library is no longer on the SD Card and to read/choose the external drive without changing the directory it is in?

Ideally I would like it so that I could use the drive on the MacBook and the Windows PC.

My MacBook is running Paragon NTFS so it has no trouble detecting and reading the drive.

The directory on the external drive under windows is M:\iTunes

Is there a simple way or am I best just to get a second drive (something I am contemplating) and use iTunes to transfer it all over to that and start fresh?

Will I still be able to stream content to my devices through Apple TV?

I hope the information is what you need.

PC: Windows 7 Home 64bit, 32Gb ram, 2Tb HDD

MAC: Yosemite (OSX 10.10.3) - 500Gb drive, 16Gb of ram (6 months old)

Thank you in advance for your advice. I have just changed from the dark side of Windows to Apple totally after previously only having an iPad, iPod and Apple TV I now have a MacBook Pro, Time Machine, and an iPhone 6+ so it is a learning curve for me, but I am loving it all. This is the first frustration I have felt since I have changed over.