What is the source that you are looking at to determine how much space you are using on your HDD? If it is the display from ABOUT THIS MAC, it is not to be trusted. there is a bug in the program.
Download from the Internet OmniDiskSweeper and Grand Perspective (both free) and open them. They will show all of your files and the respective sizes. This should allow you to determine what files to delete or off load to an external HDD.
Do not forget to empty trash. Only then is space allocated for new data.
https://www.omnigroup.com/more
http://grandperspectiv.sourceforge.net/
Ciao.