Yosamite/Mavericks + no space on startup Disk

Macbook 13" Mavericks OS X10.9.5, purchased in 2014.

I keep getting the "your startup disk is almost full" message. I have emptied all Trash folders (iphoto and desktop)

When I go to "Flash Storage" it says 121GB available... Music/photos/Movies/Apps take up about 80GB and OTHER: 34GB ... what is this??

This has become quite the nuisance as I can't upgrade my computer software to Yosamite.. Can't backup my Iphone, which by the way is doing the whole "blue/black/white screen of death".. I can only get it turned on for 10-15 minutes at a time..

Any assistance about how I can utilize those 34 GB of "other" storage would be much appreciated..

*Also, I've fully utilized Icloud and dropbox storage

Thank you!