When you work in a file, Microsoft Excel saves the file with a temporary file name (.tmp file extension) and places this file in the same directory as the original file. When you save the file, the original file is then deleted and the temporary file is renamed with the original file name. Therefore, the amount of free disk space on the drive must be greater than twice the file size being saved. If it is not, then you will receive the message "Disk is Full."
For example, if you open a 710-kilobyte (KB) workbook, and you make changes that add 10kb to the file size and save the workbook the disk must have over 720kb x 2 or more than 1.44mb of space for Excel to work with this file.