Windows 7 sometimes uses a hidden disk partition called System Reserved to store critical system information. During a registry backup, Microsoft writes temporary files to the System Reserved partition. If this partition has less then 40MB of free space, Retrospect will be unable to perform a system state (registry) backup of the computer.
You can verify the amount of free space on that hidden partition by viewing your disks inside Windows 7 Disk Management:
Go to Start and type in diskmgmt.msc and the program should appear above. Click the program to launch it or hit the Enter key.
Under Volume, right-click on the System Reserved partition and select Properties.
The free space of the partition will appear as the pink part of the graph.
You can increase the amount of free space on that partition with the following steps:
In Disk Management, assign a drive letter to the System Reserved partition.
In the command prompt (running as Administrator), run:
FSUTIL USN DELETEJOURNAL /N /D Z: where "Z" is the new drive letter.
In Disk Management, remove the drive letter from the System Reserved partition.
Make sure there is enough free space on each volume for backup to run. Backup requires some additional disk space on the client computer to create a VSS snapshot. On any volume that is not system reserved, at least 10 percent of available disk space must be free. On a system reserved volume, if the volume size is less than 500 MB, VSS requires 32 MB of free space to create a snapshot; if the volume is 500 MB or greater, VSS requires 320 MB of free space. For more information, see this Microsoft article.
Last Update: December 17, 2014