It can be a confusing scenario when you are working heavily on a document and you receive a message that your disk is full when trying to save your work. This is especially concerning when you are working on something important and you know for certain that your disk couldn't possibly be filled. This is an error that occurs when you regularly save your work at an excessive rate, and resolving the problem for the future is easy.
Check your computer and make sure your disk isn't actually full. The more you add to a document, the larger it becomes. There is always the chance the change in size is actually more than your disk can handle. Open "My Computer" and look at the bar beneath the drive where your file is located. If the bar is red, the disk is likely too full to store any more. Move some files to another hard drive or external drive or delete files.
Monitor your saves. This error message is likely to appear when you have saved a document an extreme amount of times. If file sharing is turned on, the limit for saving a file each time you open a document is 60 times. Close and reopen the document every 20 saves if you must keep file sharing active.
Disable file-sharing if you don't need it. Click "File Sharing" in "Control Panel" while your document is open. Click "Start/Stop" and click "Stop." This will remove the limit of saves you can make, causing the error to stop.