I have a mid 2010 MacBook Pro currently using MacOS X El Capitan 10.11.6. I downloaded MacOS High Sierra 10.13.4 and have the "Install MacOS High Sierra" application. My problem is that when I try to install MacOS High Sierra, the only available disk to select is "Restore Image," which "Install MacOS High Sierra" tells me is used for Time Machine backups, but it is gray and faded and I can't select it. I have FileVault turned off and "Restore Image" selected in my "Startup Disk" menu. How can I be able to select the disk and install MacOS High Sierra 10.13.4?
I think the installer is not allowing you to install because you need to FIRST update the firmware. You do this by making a USB installer from the installer app and booting to it.
You will need to choose HFS+ as the format and the High Sierra Installer will convert the disk to APFS (if the target drive is an SSD).