Mac: Seagate external drive won't eject, in use

Did you install the Seagate Toolkit software...?

If so, then you need to uninstall the Seagate Toolkit.

see > https://www.seagate.com/support/kb/uninstalling-toolkit/

Did you install a third-party Time Machine utility...?

If so, then you need to uninstall that third-party Time Machine utility.

If the answer to both is No, try pausing Time Machine backup and then ejecting the Seagate drive.

Then remember to resume the Time Machine backup when you reconnect the Seagate drive.

see > Start, pause, or resume a Time Machine backup on Mac - Apple Support

macOS has a hard time releasing its hold on mounted volumes. Sometimes if you click unmount/eject a second time it will let go (best to wait at least 15-20 seconds, perhaps more).

You can also try using the command line to unmount all the volumes on that drive as it may tell you why it could not be unmounted or ejected.

Get the device identifier for the physical external drive from Disk Utility. You may need to click "View" and select "Show All Devices" before the physical drives appear on the left pane of Disk Utility. The device identifier will appear in the format "diskX" where "X" is a number....for example "disk4".

Open the Terminal app and use the following command template making sure to replace "diskX" with the proper device identifier:

diskutil  unmountDisk  diskX

In my example:

diskutil  unmountDisk  disk4

Keep in mind the device identifier can change every time you connect the drive, so you must always check for the current device identifier.

When you got the drive, did you completely erase and reformat it using Disk Utility, or did you use the setup tools that were preinstalled on the drive?

Is the drive indexed by Spotlight? If so, you can exclude it in System Settings > Spotlight, click the Search Privacy button at the bottom of the settings pane, then click (+) and add the drive to the exclusion list.

As den.thed stated, If the drive is used as Time Machine you need to stop Time Machine.