1TB External HD not recognised after connecting to android

I can’t access the HD in file finder on mac, after I connected it to an android notebook.
It shows up in disk utility as ‘disk3s2’, says its not mounted, but I can’t access the files.
It was formatted to FAT so it worked on PC & Mac, when it connects to PC (win7) it prompts me to format the HD.

Does anyone know what I could do to gain access to the HD and back it up to another 1TB WD HD? Many thanks in advance.