A .pst file backup fails even when there is sufficient free space on the destination disk on a Windows Vista-based or Windows Server 2008-based computer that has the Pfbackup utility installed
Consider the following scenario. On a computer that is running Windows Vista or Windows Server 2008, you have Microsoft Outlook and the Microsoft Outlook Personal Folder Backup utility (Pfbackup) installed. Then, you run the Pfbackup utility to back up the personal folder file (.pst file) to a disk. In this scenario, you receive an error message that indicates insufficient free space on the disk for this backup operation. However, when you check the free space on the disk, you find that there is sufficient space for the backup if the old .pst file is overwritten as expected.
For example, assume that you have a disk that contains 100 megabytes (MB) of available free space and a 200-MB copy of the backup .pst file. When you try to perform a new backup for the .pst file on this disk, you receive a “Not Enough Space” error message. However, you expect the old .pst file to be overwritten during the backup operation, and this should produce sufficient free disk space for the backup operation.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB960711.