I purchased a 1TB GoFlex Drive several years ago. It said it was supposed to work with both Mac OS X and Windows PCs. Unfortunately, I forgot that you need to install a piece of software to get it to work simply on a Mac.
Or reformat into a format that works for both platforms seamlessly.
If you’re looking for it. It’s called Paragon Driver for Mac you can find it here.
Hopefully this will save some time and heart ache for you.