Note: Reformatting the drive will erase all data on the drive, so you should copy any data you want off the drive prior to formatting.
The following is based on the latest version of macOS.
1. Open Disk Utility
Open Finder > Applications > Utilities > Disk Utility
2. Select the Seagate drive on the left.
Note: If you don’t see two entries listed then change the view in disk utility to show the drive and the volume. The view button is in the upper left-hand side, change to Show All devices. This feature was introduced in macOS 10.13 and not available in 10.11 or 10.12.
3. Select Erase which opens the following window.
4. Rename the drive
5. Select OS X Extended (Journaled) for Format
6. Select GUID Partition Map for Scheme
Note: If you do not see the scheme option, then you have selected the volume and not the drive. Click on the view button in the upper left-hand side, change to Show All Devices. Select the non-indented entry for your Seagate drive.
Warning: Formatting the drive will erase all data on the drive, so you should copy any data you want off the drive prior to formatting.
7. Select Erase again.
8. Disk Utility will start formatting the drive.
9. Once complete, click done.
For additional help see: Disk Utility Errors and Questions