A911Pro wrote:
I created the USB bootable installer with UltraDMG with the El Capitan OS I downloaded from Apple.
I don't know how UltraDMG makes the El Capitan USB installer, but I'm certain it is a bit of a hack. It doesn't help that the UltraDMG page has no useful information about their app in this regards. I'm not saying it cannot work, only that it probably is missing some components that may be critical in some situations. I actually have some personal experience with manually creating a bootable macOS El Capitan USB installer so I am aware of some of the underlying details & methods used to make a bootable macOS USB installer.
The best most reliable way to make a bootable macOS USB installer is by using the instructions in the following Apple article using a computer compatible with the OS installer you are trying to create:
Create a bootable installer for macOS - Apple Support
What exactly happens when you attempt to boot from the El Capitan USB installer?
No firmware password. See original post for where we are getting the authentication issue. The errors very, sometimes as I described above and other times Apple would say it couldn't send the code. The errors always occur after logging in.
That was not clear in the earlier posts. It sounded like you were getting these errors while attempting to either boot the installer or during the installation attempt. Since we cannot see your computer we need very clear & detailed information on what happens....including exact error messages.
On an erased computer, the only times you would be presented with the need to authenticate with an AppleID is confirming a previous purchase of macOS El Capitan. You should be able to avoid this need to authenticate by booting into Recovery Mode using Command + Option + R.
You should not be needing to authenticate at all when using a bootable macOS 10.11 El Capitan USB installer (or when creating a bootable macOS 10.11 USB installer).
If booting into Recovery Mode using Command + Option + R does not work either, then your best option to install macOS on this iMac again is by using your OS X 10.6 Snow Leopard DVD. Depending when your Mid-2009 iMac was made, this may be the original gray Restoration DVD which shipped with the computer from the factory, or it may be the Retail Upgrade DVD which you purchased later on if the iMac was an earlier model. Once OS X 10.6 has been installed, then you can transfer the downloaded El Capitan DMG file to this iMac to extract and install macOS 10.11 El Capitan. This all assumes that the internal hard drive is not failing, otherwise even this will not work.
Here is an article with instructions for properly preparing the internal drive which requires partitioning & formatting the drive prior to installing macOS onto it:
https://eshop.macsales.com/tech_center/formatting/Mac_Formatting_6-10_R3.pdf
You can try running the Apple Hardware Test by booting from the second DVD which originally shipped with the iMac from the factory to see if any hardware issues are detected. Unfortunately the diagnostics rarely detect issues even when they are confirmed to exist through other means, but it never hurts to check.
I think you may need to have someone local who is knowledgeable about these older iMacs to look at the computer & assist you. More than likely you have a hardware issue of some sort (most likely a failing hard drive) which may be complicating things in addition to perhaps not having a properly made macOS USB installer.
FYI, the mid-2009 iMac does not support Internet Recovery Mode, only local Recovery Mode.
Info from sticker: iMac 20" M2009 4GB RAM 160GB HD.
Your link simply says coverage expired when entering the serial number, no further information.
Seems Apple modified their website again to exclude older systems which is very unfortunate. The information on the sticker tells us you have an iMac 20" (Mid-2009) model. @BDAqua does provide another site where the model information can be acquired.
