I have a Mac Book Pro laptop. When clearing it of personal data etc ready to sell on when I brought my desktop iMac I appear to have erased the start up discs etc. I have used the online apple support features showing how to restore but they don't work. When I try to reinstall from the menu once it is booted up I get the message UNTRUSTED_CERT_TITLE and can't get any further. I've gone through the disc utility first aid functions etc and have attempted to choose my start up disc but just get the message 'You can't change the startup dose t the selected disk. Startup disk could not gather enough information on the selected ideas' ( would attach the photos I took but for some reason the computer won't allow me to!). Has any one any ideas of what to do next – and if it's getting apple to sort it any idea how much it will set me back? I don't have an awful lot to expend on it.