I know you've already had a lot of help on this, but I thought I'd just let you know the causes that I've had with this problem in the past.
I've sometimes been working on a workbook opened from a network drive, that gives the disk full error message when trying to save ( many gigs free on the network drive, megabytes free on the PC ). Once the message has appeared ( usually followed by another message saying Document Not Saved ), then I find it is impossible to save the workbook anywhere, under any name, having tried different network resources, different folders on my PC, even tried a floppy, but with no success.
The cause was found to be network polling slowing down to a point where Excel "lost" the connection ( I know this sounds ridiculous, as the drive is still visible and other files can be opened and saved while all this is going on ), and then, for the particular file that is open, Excel not knowing how to cope with it.
The only way to try and rescue any work that's hanging like this is to try to copy ALL items from the hanging workbook into a new workbook, and then save that.
Glenn.