There will be two hidden partitions taking up space
One (the largest)is for the advanced tools and factory reset images
the second (quite small) is part of the new UEFI /GPT BIOS system which gives the machine details of the hard drive and bootable systems that may be on it.
Do not interfere with these partitions
If you make images of your system (recommended)to an external source, also make sure it images the whole disk not just C drive then if the hard drive fails then image can be put on to a new hard drive without problems occurring when changing the drive.