|Check the source and destination disks for errors with Windows error-checker:|
Right-click the drive in 'My Computer', select 'Properties'.
Click 'Check for errors', tick both options, click 'Start'.
For drive 'C' you'll need to restart Windows before the check will start, as 'C' can't be checked while Windows is running.
If that makes no difference and one or both drives is external, try using different usb ports.