Upgrade to High Sierra "is damaged, and can't be used to install macOS" in the App Store
I'm trying to upgrade my late 2009 iMac 27" from Sierra to High Sierra. When I find the High Sierra page in the App Store, I click "Open" and after a pause get a message: "This coy of the install macOS High Sierra application is damaged, and can't be used to install macOS." What options do I have now to upgrade to HS?
iMac Line (2012 and Later)