Hi, a friend brought round his mac book pro which was stuck in recovery mode and would not install the os (yosemite). He said he wanted it wiping as he was going to give it to his son. I created a bootable el capitan usb disk using my mac which seemed to work fine, the mac saw the drive and the install seemed to be working fine, it got to where the installer says "about one second left" but then failed and showed the error "This copy of the Install OS X El Capitan application can't be verified. It may have been corrupted or tampered with during downloading". Now it will not start up, when the usb drive is plugged in the apple logo appears with the loading bar and this gets about 3/4 and then stays there, without the usb drive in i get a folder with a question mark flashing. I cannot open the mac in recovery/safe mode and when I open holding option i only get the option to select the usb drive (no mac hd) .. when i choose the usb drive it just gets stuck on the apple loading screen again. Any help would be much appreciated.