How do I find out what applications are using my SD Card. I want to eject it safely rather than force eject.

I am on Mac OS X 10.9.5 and have been using a Micro SD Card for my music to add to my phone. Up until now it has been fine. It has started saying I that some applications are using the SD Card so I can't eject it safely. It only allows me to force eject, but I don't want to do this incase I lose songs/data on card. Could anyone help me find out how to safely eject it or how to find out what applications are using the card and how to quit them? I definitely don't have any obvious applications using the card so it must be something hidden. I've had to keep the card in for a while because no forum i've seen on google has given a helpful answer. I'd really appreciate any help.