Can't save photoshop file to dropbox or hard drive - receiving scratch disk error message

Hello, 

I've been trying to save .psd files to my computer (tried on my main drive and dropbox) but I keep encountering the following error message: could not save "file name" because the scratch disks are full. I have cleaned up my Mac HD disk and made sure there is suitable storage space (more than 10GB available) but this doesn't seem to resolve the issue. Is there anything else I can do troubleshoot this?  

Thanks in advance.