MACOS installation failure. "An error occurred while installing firmware. ... Restart your computer and try again." Attempted restart...

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Hi Robert,

Are you trying to reinstall the operating system of your imac? 

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Hi Robert,

Are you trying to reinstall the operating system of your imac? 

May i know the exact model of your iMac? there are cases that the upgrade will not push through because the system cant handle the new operating system or due to system limitations. 

I see. So you are trying to upgrade the operating system to High Sierra? is that right? 

I see. Seems like it upgraded automatically to Mohave. You can try to revert to the old operating system. Please try this steps below

1. Start up your Mac while holding down Command+R until you hear the startup chime and see either a spinning globe or Apple logo.
2. A macOS Utilities window will open, on which you will see the options to Restore From Time Machine Backup, Reinstall macOS, Get Help Online and Disk Utility. Your next step depends on whether you wish to wipe the Mac before reinstalling the operating system. If you want to wipe your disk click Disk Utility (otherwise you can jump ahead to point xx).
3. Having selected Disk Utility, click Continue.
4. Now select the volume you wish to Erase (likely Macintosh HD) and click Erase (you may need to click on View > Show All Drives before you can see the volume). Remember this is going to delete all your data so make sure you have made a backup. 
5. Make sure that the Format is set to Mac OS Extended (Journaled) and that if available you have GUID Partition Table selected as the Scheme.
6. Click Erase and wait. (There are options under Security Options that allow you to erase the disk securely).
7. Now choose Disk Utility > Quit Disk Utility.
8. You'll see the OS X Utilities screen again, choose Reinstall macOS.

Are you still there Robert?

I am trying to call you on your phone but no one is answering the call. Were you able to read the instructions that I provided to you below?

if you dont want to wipe the mac before reinstalling means you can either go to Restore From Time Machine Backup or Reinstall macOS.

if you dont have a time machine backup then the only way is to reinstall the OS. Please take note that this will wipe out the operating system on the computer and will replace with a fresh one. 

Can i call you on your phone? 

yes you can use the time machine backup to restore the imac to old OS and that will restore also the original files to your imac. 

Do you have further questions? 

Dear Robert, It was a pleasure helping you with your iMac issue today. Here is a summary of the resolution.
Issue identified : cx getting error while installing the latest ios upgrade on his imac
Solution Implemented : chat with cx and told him that case you are getting an error is because the ios is not compatible to the machine due system limitation. Advised cx to revert to the original ios. Provided cx steps (steps below) on how to revert the machine. cx said he has time machine backup. told cx that he can use time machine backup to restore the machine. cx will perform the steps provided. answer accepted. 
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