Fixing Mac OS X Install Errors “can’t be verified” and “error occurred while preparing the installation”
Two unusual error messages that can occur during the attempted installation of Mac OS X El Capitan or Mac OS X Yosemite are the “This copy of the Install OS X El Capitan application can’t be verified. It may have been corrupted or tampered with during downloading” error, or a “This copy of the Install OS X Yosemite application can’t be verified. It may have been corrupted or tampered with during downloading” message, or a more vague “An error occurred while preparing the installation. Try running again” error message. Sometimes these can be resolved by simply rebooting and trying to reinstall OS X again (or re-downloading the Mac OS X installer if it was damaged), but if the error messages are persistent, then you may find that modifying the system date of the Mac can be the resolution.