Chosen Solution
Good day I have a 2008 imac 20in with a working El Capitan. Because i was planning to sell it I re installed the OS, but it’s giving my the error installer not recognize, damage etc. I have tried: change date(wifi off) reset the PRAM, removed the battery and still having the error when installing. The USB I’m using is working, I setup an iMac prior to this unit and I created another USB just to be sure but still having the error.
Ah! You’re hitting the expired certificate issue within the installer. Here’s more on this issue: If you’ve got an old macOS install image, it will probably stop working today From this mess Apple did update many of the older OS’s. To get a good version download from here How to get old versions of macOS I recommend setting up a USB thumb drive as an OS installer, First reformat the drive to GUID Journaled using Disk Utility then follow this guide How to make a bootable OS X 10.11 El Capitan installer drive. Then just boot off of the installer to first fully wipe your drive again use Disk Utility wiping it with a pattern so your data is not recoverable. Then use the the OS installer to install a fresh OS.
