This happens to me quite a bit. I use Microsoft power toys and they introduced a new tool called file locksmith. Basically, you right click a drive, file or folder select more options and click what is using this file. Software will tell you exactly what is open for that item you selected.
Solution:
Go to windows store and download power toys from Microsoft. This app has a bunch of useful tools to help make windows better. The tool you are looking for is file locksmith. Right click on the external device and select more options. Click on what is using this file. File locksmith will open and after a few moments will display the open services being in use. Click end on each service then eject again. If after a few minutes and file locksmith is still searching it means nothing is running. Try to eject again and it should work.
