If you get the error message "Disk Error - Error writing file; check disk space" you may also notice that in the message window you get a message "Write access denied to <OBJECT_NAME>".

If, however, you know full well that the disk is not full, then this error message is misleading.  The real problem is that you do not have write access to the folder from which <OBJECT_NAME> was retrieved.  This is often associated with objects retrieved from PDM-links.

The quick way to get out of this mess is to use File>Backup which saves all dependencies to a specified folder, regardless of whether they have not been modified or which folder they were retrieved from.  But beware, you may end up saving a lot of files - you may even run out of disk space for real!  You will then have to sort through all the saved files and work out which ones you need to keep and which ones can be thrown away.

To take a more sensible approach, open up <OBJECT_NAME> and use File>Backup to save this file to your working directory.  Repeat for each object which cannot be saved.  Then you should be able to save your main assembly without problem.