External drive won't eject cleanly
This issue is generally caused by a background process that is taking a (very) long time to complete. 

In my case it was Spotlight indexing, which I don't need for the drive in question. So after confirming that it the culprit was indeed Spotlight and terminating it, I simply disabled Spotlight indexing for the drive, and now I am able to eject cleanly. Similar instructions apply for identifying and terminating whatever process is the cause generally.