it's because you don't have enough space in the hidden system reserve partition, first can you make us a print screen of your disk management...right click on computer icon > management > disk management > right click on the reserve partion and show us property like in my image
Hi
Use Macrium "Reflect" free version (Uk macrium website, check google).. it works in Windows 8 and is far better than Windows own version
Best to have a spare partition or drive to back up to .( 20 gb upwards )