Time Machine just stopped backing up to my Seagate Hub, how to fix it?

Check the External Hard Disk using DriveDX and see if it is dying, that would explain it. However, first restart the computer in Safe Mode then restart normally and re test. Finally, heck out Time Machine settings to ensure the External Hard Disk hasn’t been excluded.

Open Time Machines settings (while in Time Machine type Command key and “,” key and ensure the External Hard Disk is not excluded.

Your welcome.