Hi Dean, Welcome to Microsoft Community and thank you for posting your query. Let me help you to fix the issue. I suggest you to install the drivers supplied by the Seagate manufacture any disc or USB, which came along with the device and check. If you do not have the disc or USB, visit the below link and try downloading the software and install the same.
Seagate Goflex Software For Mac
I would also recommend you to go to the Disk Management and check if it is detecting the disk drive. Press Windows + R and type diskmgmt.msc on the box and hit Enter. Hope this helps. Keep posting us with status we will be glad to help you.