Unable to eject Android phone

As Andrew Halloran stated, device can be safely removed if you first end Android Debug Bridge adb. That can be done using the Task Manager

Then click "Safely Remove Hardware and Eject Media"

Android uses ADB to communicate with physical devices and emulators. To properly disconnect your phone first stop the ADB server and then eject your device.

CD C:\Users\<user_name>\AppData\Local\Android\android-sdk\platform-tools
adb kill-server

Then eject your device.

More info here.