I thought this was simply a matter of choosing Master Boot Record for the format in Disc Utility. I just did that with a USB flash drive and a Windows user was unable to read it. (A tax service business. With 'working' Windows machines. I didn't dare ask him to try downloading anything to try as I know those machines are easy to brick.) So... How do you format a drive volume so a Windows user (with an out of the box setup and no computer skills) can read it?