disc not ejected properly

Below is the link with instructions on hwo to do a Safe Mode restart:

Safe Mode Reset

First Aid is a tool within the Disk Utility app (Applications - Utilities - Disk Utility) , you can use it for gauging the health of an external drive. If you ever need to check the health of the internal drive on your iMac you would need to restart in the Recovery Partition. To get to the RP please hold down the Command + R keys on start up. When you run First Aide on any drive run it several times in succession and review the report and look for any errors. If errors come up that is a warning a drive is likely failing.

As you are unfamiliar with DU and First Aid, I would recommend navigating to youtube.com and look for some instructional videos.

Please restart in Safe Mode and then restart normally and test. If the problem continues run First Aid on the disk in question.