I removed Macintosh HD from backup disk how do I set it back?
Okay, now that we have clarified what the issue is, here's how to solve your problem.


Open System Preferences
Select "Time Machine"
Click "Options"
Macintosh HD will probably be listed under "Exclude these items from backups:". Click it and click the "-" button below the box and then click "Save"
Make sure that Time Machine shows the correct disk in the window (in your case, it should say "Seagate-3E59C4 TM Backup - Seagate-3E59C4". It is important to note that "Macintosh HD" will not show up here as it is displaying the disk to be backed up to, not the disk that is being backed up. "Macintosh HD" should never show up when you click "Select Disk..."
If you are running a MacBook, plug the MacBook in to power
In the menu bar, click the Time Machine icon and select "Back Up Now"


Time Machine should now start backing up to your Seagate drive. If you are still having trouble with Time Machine looking for the disk, then do the following:


Click "Select Disk..." in the Time Machine preferences
Select "Seagate-3E59C4 TM Backup" under "Backup Disks"
Click the "Remove Disk" button. The dialog will prompt you for confirmation and close
Click the "Select Backup Disk..." button
Select "Seagate-3E59C4 TM Backup" again and click "Use Disk"
If your Seagate is password protected, it will ask you for the password