If you recently upgraded to Mac OS Sierra 10.12.3 or 10.12.4 and found out that your Seagate External hard drives are not being detected by your Mac anymore, the fix is pretty simple. All you need to do is install a new disk driver from Seagate.
How to fix Seagate external hard disk not working on Mac issue?
Simply download the latest available driver from Seagate’s official website. The Paragon driver will immediately allow your Mac to detect the Seagate external hard drive.
http://www.seagate.com/in/en/support/software/paragon/ is where you will find the driver. Make sure you download the Paragon Driver for Mac OS (10.9 and above), the second option listed in that page.
If that doesn’t work, you must also try other common culprit issues by swapping out the disk cable and also by trying another USB port on your Mac. Also, try restarting your Mac in safe mode to see if that will fix the problem. You can also try what is called a SMC reset or a system controller reset to see if that will fix your undetected Seagate external disk issue.
Please remember that your Seagate external disk will also not work on your Mac if it uses the NTFS format (used in Windows). You will need the disk to be formatted in HFS+ format.