|It may simply be that Windows has not assigned a drive-letter to it, and a drive that has no drive-letter will not be visible in Windows Explorer. There is a simple fix if that's what has happened:|
Open Control Panel > Admin Tools > Computer Management > Disk Management
Look in the bottom-right pane at the horizontal ribbons representing the drives.
If you see a ribbon that shows the capacity of your seagate external drive, but without a drive-letter, right-click on that ribbon and choose Change Drive Letters & Paths
Choose a drive-letter.