QUICK LOOK | Disk Utility
Disks and volumes
The left pane shows disks and their volumes. Click the disk to erase it, format it and create a new volume for El Capitan.
When the disk is highlighted, you’ll see the Partition tab. If you click a volume in left window, the Partition tab isn’t visible.
The bottom of the window displays information about the selected disk, including its capacity and partition map scheme.
Partition map scheme
With your disk selected, make sure this says ‘GUID Partition Table’ so that El Capitan will boot.