My iMac was very slow so I decide to format it from a usb dongle containing High sierra and to reinstall the system. I was able to format the hard disk to verify it (disk utility said it is ok) but when I tried to install from my dongle I have the following message : “This copy of the Install macOS High sierra.app application is damaged, and can’t be used to install macOS.”
I know my dongle is Ok because I test it on another computer. I also tried with a Mojave dongle and I have the same message.
Why my imac is not letting me installing a system from a dongle ?? Now my hard disk is empty and I'm stuck. Is it a hardware problem ?