Are you sure that it wasn't just your library being updated? Sometimes it takes a while.
If its not that, you may have a compatibility issue. One of your pieces may not support a file system that the other does. (or something similar.). Get a usb stick and format it to fat32 or exfat. Then put some music on it from the problem library drive and see if it will play from that.
I haven't used a Mac in a while, so I'm not sure, but you may need to mount your drives/partitions. You can't access anything that's not mounted.
If its not that, you may have a compatibility issue. One of your pieces may not support a file system that the other does. (or something similar.). Get a usb stick and format it to fat32 or exfat. Then put some music on it from the problem library drive and see if it will play from that.
I haven't used a Mac in a while, so I'm not sure, but you may need to mount your drives/partitions. You can't access anything that's not mounted.