disk partition, even if you are using it at the same time you want to clone it. You can save the resulting disk image inside the same partition, or on a network, portable drive, or CD/DVD.
The algorithm used by Macrium Reflect Free reduces the size of the disk image drastically, and still achieves an optimum speed. Obviously, you can recover the disk copy right from Macrium Reflect, and even create recovery CDs for Linux.