2012 iMac boots to prohibited sign after it attempted over night self update? HDD now cannot be mounted? My friend's 2012 iMac attempted to self update over night and it resulted in the prohibited sign on a gray screen this morning and so, the iMac does not boot anymore!

Here are all the things I've tried so far:

1) Holding down SHIFT on power up to go to safe mode, results in black progress bar followed by prohibited sign.

2) Holding down CMD-R on power up to go to the recovery console, results in black progress bar followed by prohibited sign.

3) Holding down CMD-OPT-R on power up to go to the online recovery console, this works, it connects to the WiFi and boots into the recovery console successfully.

3a) From that online console, I can go to Disk Utility and I see the internal 1TB HDD, but I cannot mount it as it does nothing when selecting the MOUNT item.  Also, that HDD (disk0) information is very minimal like the drive was inaccessible or something.  However, from the Terminal > "diskutil list" command, I can see two partitions inside disk0, but nothing happens when I mount or force mount any of them: disk0s1 (150MB EFI) and disk0s2 (1000GB APFS)

3b) In Disk Utility, I tried to use the First Aid functionality on disk0 (not the actual partitions since Disk Utility does not show them, but rather the root disk element itself) and it's been sitting there in a popup window displaying "Running First Aid..." without any sort of progress for an hour now.  It does not list or show anything happening in that window and there is no cancel or close button either.  I forced a power down on this one as it was not seemingly working at all.

4) Holding down SHIFT-CMD-OPT-R on power up to go to the online factory reset console, this works, it connects to the WiFi and boots into the recovery console successfully.  In Disk Utility, I GUID partitioned an external 500GB USB HDD and formatted it as HFS+ then proceeded to install Mac OS X Mountain Lion (10.8) which is what the factory reset console suggests and what my friend says the iMac came with in 2012.  It took 3.5 hours and it worked.

4a) I understand that APFS is only available since High Sierra and not Mountain Lion so installing Mountain Lion as per described in 4) is useless even tough it worked, because obviously, the HDD wasn't mountable using 10.8.  So I opened the iMac and replaced the internal hard drive with a brand new one.  I am now attempting the same as 4) with 10.15 Catalina instead, which is suggested when booting the iMac via the online recovery console (CMD-OPT-R) anyway. I am sure this will work (3 hours left to install) but since I want to get the DATA back from the old drive, I'm eager to see if connecting the older drive via USB will make it mountable after booting 10.15 Catalina via the brand new internal hard drive.

EDIT: So, this morning, Catalina has finished downloading/installing (4a, above) on the brand new internal HDD that I've installed yesterday evening.  So the iMac works flawlessly now, and thanks to iCloud syncs, almost every document is back in the computer, it's still downloading documents right now so I'll wait until it's all done (is there a way to know when an iCloud sync back is completely finished anyone?).  The old internal drive, which I've attached via USB DOES NOT MOUNT, still. And Disk First Aid still seems stuck on diagnosing the drive! So it may just be dead, like you mentioned. I'll leave Disk First Aid run for 2 hours and I'll report back afterwards.

I am aware of those 2 interesting ideas, however like I mentioned, we cannot access disk0s2 to browse/delete files.

System won&#39;t boot after attempting 10.13.4 update

OSX Prohibited symbol on Boot (High Sierra Update Caused)

Any more ideas? We do NOT have another Mac, this is the only Mac we can use.  However, I have some external drives that we could format and use.  Also, internet is very slow here (booting to online recovery console begins by downloading like 1GB of DATA and this takes almost half an hour!) so the most preferred method would not imply downloading gigabytes of install files, unless it's actually mandatory.
There are two possibilities. There have been reports of Catalina causing some corruption to the EFI firmware partition. However, if the iMac was running auto-updates, then it should have already updated to Catalina? In which case that's probably not the case.

The other possibility is that the 7-year-old internal hard drive has failed. The computer itself seems to be working as you've booted to an external. 

Drive failure often happens to old drives after an OS upgrade ("The new OS broke my drive!") because writing several GB of files to the drive is the 'final straw that breaks the camel's back'.  

It's possible that the drive may mount and be read once you've booted to a Catalina OS. You might need to run third-party disk recovery software like Data Rescue.

However, if the drive will not mount at all, then data recovery becomes much more difficult. And by difficult, I mean expensive.
This is where I ask you if you have a backup.