My Book Live Duo Format?

How does the NAS disc work with Mac and PC at the same time? Is it FAT32 format, so u can´t backup files above 4gb? 

It’s a NAS.  

You don’t need to worry about the filesystem format on NAS boxes;  the PC is unable to tell anyway.

But if you’re curious, the filesystem is EXT4, and has no problems with huge files.