Macbook startup disk full; GP shows 164GB of 'Miscellaneous used space'
I found the this link on MacRumors that sounds like the same problem you are having. The gist of it seems to be clearing the system cache using this command sudo rm -r /System/Library/Caches could help ( assuming that is where the issue lies). You should also be able to hopefully figure out what is using up all the space with this command sudo du -chxd 1 / to see what is taking up all the space.