A difference in expected space utilization and also file and folder counts between the source and destination drives can be caused by a number of things, including the Windows swap file, hibernation file, and other temporary and instance specific files.

Casper actually excludes the Windows swap file (pagefile.sys) and hibernation file (hiberfil.sys) as an optimization when cloning a drive on which Windows is running. The swap file is a temporary file used by Windows to manage virtual memory and is created on demand when needed. Likewise, the hibernation file is also a temporary file used by Windows to hibernate the computer and is created on demand when needed. Depending on the amount of RAM installed in the computer, both of these files can be extremely large. The size of a hibernation file is typically equal to the amount of RAM installed. The size of a swap file is typically much greater than the amount of RAM installed (2 or 3 times the size is not atypical).

In addition, Casper will exclude certain files within the Windows System Volume Information and Prefetch folders when necessary. Primarily these files represent system restore points (persistent volume snapshots) and data specific to the original disk configuration which should not be included. These files can represent a considerable amount of space – e.g., many gigabytes.

Other observable differences may include possibly new or changed files in the logged-in user Temp folder and Windows temp folder. Depending on your system setup, services and utilities running under Windows may be changing, creating, and removing temporary or state files so it may be impossible to find an exact byte utilization match on an active system drive. When cloning a drive, Casper actually begins by taking a snapshot or "picture" of the drive and then clones the snapshot. This ensures a stable, point-in-time image of the drive is cloned.

If Casper has any trouble during the cloning process, it will log the problem as well as any affected files and/or directories in the Exceptions section of the Activity Report that is generated at the completion of the cloning process. Casper will also alert you to these exceptions at the completion of the Copy Drive wizard. To review a report generated by Casper, follow this procedure:

1. Open Casper Explorer
2. Click the History button on the toolbar (or from the View menu, click Backup History or Explorer Bar -> History depending on which version of Casper you are currently using) to open the History navigation panel
3. In the History navigation panel, expand the folder representing the day in which the copy was performed
4. Display the Casper Activity Report for the copy by clicking on its date/time. (This report will be entitled "Casper Activity Report".)

To determine the amount of hard disk space consumed by persistent volume snapshots created as part of the System Restore and Previous Versions functionality provided by Windows, follow this procedure:

1. Open an elevated Command Prompt (On Windows 10 or later, right-click the Windows Start button and then click Terminal (Admin), Command Prompt (Admin) or PowerShell (Admin). In Windows 8 and later, point to the lower-left corner of the screen, right-click the preview or Windows Start button, and then click Command Prompt (Admin). In Windows 7 or Vista, click Start, All Programs, Accessories, and then right-click Command Prompt and select Run as administrator.)
2. At the command-prompt, type vssadmin list shadowstorage and press Enter

You should see something similar to the following reported:
 
As this screen shot shows, this system is using approximately 66GB of disk space for shadow copies (persistent volume snapshots).

• Windows 11
• Windows 10
• Windows 8
• Windows 7
• Windows Vista