MBWE + iTunes = exclamation marks

WD Legacy Products Other Network Drives
1

I’ve got a MBWE 2 TB network drive, where my iTunes library is located. However, the entire library has exclamation marks next to the individual songs. An acquaintance of mine told me that the problem is with my network drive, and that the drive does something to the music library every time it runs automatic backup. 

Any help understanding this?