Airport Extreme - USB Disk as network disk for Windows and Mac, what format?
From https://support.apple.com/en-us/HT202807


  Supported formats: Mac OS Extended (HFS-plus), FAT16, FAT32
  
  If HFS+ formatting is used, AFP and Microsoft SMB/CIFS protocols are
  used to share the device on the network. If a FAT16/32 format is used,
  only SMB/CIFS protocols are used and AFP won't be available.


So if you are using FAT32 and connecting to it from Mac and Windows using SMB it should work fine.