How to take backup of a partition from another partition I have two partition one for Yosemite OS as Macintosh HD  and another for El Capitan OS as El Capitan

Now El-capitan part is not booting up and I have to take a data backup of El-Capitan from Yosemite so that I can reformat the El-Capitan partition and later restore from the back up.

I couldn't find any tools that does this.

Any pointers to achieve this is highly appreciated !



Thanks
I can think of several ways of making a back of your El Capitan partition, provided the El Capitan partition still mounts when you're booting up from Yosemite partition then you could use Time Machine to select the El Capitan Partition in System Pref/Time Machine.

Or copy the entire contents of the El Capitan partition onto an external hard drive when you boot from Yosemite.