Factory Restore the device?

Hi

My ixpand drive was formated by mistake and is not recognized any longer by my iPhone 6s (on ios11).

I 've tried to restore it ( in exFAT format) using diskutility on my Mac but I can’t find this MBR for the partition type. It is not listed under ‘partition’. I am on macos high Sierra latest version. 

Thanks for any help