Can't eject external USB hard drive in Windows 10

I believe the website which you are referring to already answers your question.

Basically its an user interface inconsistency. For external hard drives which register themselves as "fixed" drives (compared to "removable"), Windows does not show the "Eject" option in the context menu when you right-click on these drives (e.g. in File Explorer or Disk Management). It has nothing to do whether a drive is configured for quick removal.

As you noticed, you should be able to eject these drives by clicking on the “Safely Remove Hardware and Eject Media” icon in the system tray and selecting your USB drive to eject it.

However, I figured that "removability" might depend on the USB ports used. For example, I can eject "fixed" USB drives when they are connected to the integrated USB 2.0 ports but cannot eject them when they are connected to the USB3.0 ports on the installed ExpressCard.

I was having the same problem right after installing Windows 10 but rebooting solved it. Now the option to eject is available on the taskbar.