Sorry for my late answer.
Your previous post: Disc Utility to erase free space bring nothing : the free space is free. You should verify your disc to see if there is some problem in the file system.
From your second post I understand that you can try to delete some files. Have you already emptied the basket or you do not empty it from time to time ? There could be a lot of stuff.
Also some Apps have theyr own basket or a place to keep deleted data like mail.
Check if you have duplicates and if you can delete something or move something to an externald Disc.
Best solution whould be to change again the Startup disc with a bigger one.