An Error Occurred While Preparing the Installation. Try Running This Application Again

SOLVED: See this page for the solution: .

Here’s the trick: Qne doesn’t necessarily need to apply the patches on this page for unsupported Macs when using this installer, and a few minutes downloading the High Sierra installer through this app will potentially save hours or even days of frustrating and unnecessary effort.

Here’s why. By studying the <installer.log> file, I learned that Apple does not preflight the High Sierra installer’s setup on unsupported Macs. Even though Apple claims on their website that High Sierra supports my late 2009 iMac 10,1, and HS runs natively on my Mac, the High Sierra installer mistakenly considers my hardware to be no longer supported.

This lack of preflighting can cause a stock installer downloaded from Apple to repeatedly throw the dreaded error message in your post’s title, "An error occurred while preparing the installation. Try running this application again.” Apple’s installer can do that on some systems even if one retards the hardware’s clock date to just after the creation date of Apple’s stock HS installer file in 2018.

That’s why resetting the clock date alone may not necessarily help, even if one resets the date through System Preferences, saves the 2018 date, and verifies the date after a restart. What will help is downloading and rebuilding the installer’s files through DosDude’s patcher application, even if the additional 1GB of patches later do not turn out to be needed.

It’s important to recognize that his installer can fail to open a previously downloaded HS installer file from Apple, which is what happened in my case. But the additional few minutes spent downloading the HS installer’s component files through his application is time well spent, because his app builds a High Sierra installer that works in cases like mine where Apple’s own installers fail.