I am trying to dual boot OSX El Capitan and CentOS 7. Prior to attempting to install CentOS I'd like to create a clone of El Capitan on a partition of the same G Drive that I use for Time Machine.
I have created a second partition on the G Drive that is the same size as the the partition containing Macintosh HD on the MacBook Pro using Disk Utility. I formatted the partition using the Disk Utility Erase button twice, once using an Apple format, and once using FAT. I then tried to use Disk Utility Restore to clone the Macintosh HD to the partition. Both times I got the same error message "Source volume is read-write and can't be unmounted, so it can't be block copied".
How do I work around this?
I've tried following the instructions Apple.
Do I need to boot using Command "R" as this YouTube video suggests or is there another way?