This issue might be due to a bad sector located at the very end of volume that you are trying to backup.
However, as a workaround you can try the following methods:
A, If you save the backup files in the first 750GB hard drive, you can save the backup files in second 300GB hard drive with letter E to test this issue. However, if you save the backup files in second 300GB hard drive with letter E firstly, you can try to save the backup files in first 750GB hard drive with letter C or D.
B, Run check disk to check the hard drive
Note: Please ensure that the DVD-ROM is the first booting device.
1. Please insert the Windows 7 DVD and reboot the computer.
2. When we are prompted to press a key to boot from the DVD, please press Enter or the space bar.
3. Click "Next" and click "Repair your computer".
4. Please highlight "Windows 7" and click "Next".
Note: A computer may store more than one copy of Windows 7. Please highlight the one which encounters issues. The order of the items in the dialog-box is identical with the one on the boot menu.
5. In the dialog box titled "System Recovery Options", click "Command Prompt" and type in the following commands. Please press the Enter key after entering each command.
Note: Please replace "C:" with the actual drive letter of the system drive on which Windows 7 is stored.
6. Please let me know if any error is detected.
C: Use other backup tool to test if you can backup files on the computer properly.