intel iMac won't boot, no safe mode, no recovery mode. no hardware faults detected.

Continued.....

• Can I use an external boot drive to access the internal drive

Yes, assuming the issue with the iMac does not prevent it. Sometimes a failing internal drive can prevent booting from external media. The fact that you can run the Apple Diagnostics indicates that you can boot to some external media, however, it may not be possible to boot to macOS since macOS is not very good dealing with unexpected situations such as a failing drive producing errors or some other issue.

• Can I use an external boot drive to recover files and to put on a fresh install?

Is it even possible to recover files?

• Is it possible to access files by removing the drive inside? A pain in the neck but I'm will to try at this point.

Possibly depending on the problem. If your iMac uses a Fusion Drive, then you do not want to remove the drive as that would break the Fusion Drive setup.

Personally I would create and use a bootable Knoppix Linux USB stick so I can verify the iMac is able to boot a full OS beyond just the Apple Diagnostics is not using a normal OS. You can use the downloaded Knoppix .iso file as a source for Etcher (Mac, Windows, Linux) to create the bootable Knoppix USB stick. Option Boot the Knoppix USB stick & select the orange icon labeled "EFI". The iMac may appear to be frozen on the Apple boot picker menu while Knoppix is booting due to the Mac's limited firmware so give Knoppix lots of time to finish booting. It is very rare for Knoppix not to boot a system even if it only ends up at a command line.

If Knoppix boots to the desktop, then you can check the health of the internal drive(s) by running GSmartControl by clicking the "Start" menu button on the lower left of the taskbar and navigating to "System Tools --> GSmartControl". Within the GSmartControl app double-click on the internal drive to access the drive's health information. There should be buttons to "View" and "Save" the report. If you have more than one internal drive, then save the report for each & post the entire report here so we can review the drives' health reports.

The Knoppix option is most likely to success over installing & booting macOS since Knoppix will usually skip past any errors it encounters, especially if the internal drive is failing since Knoppix does not try to automatically mount the internal drive unlike macOS.

• Can anyone point me as to exactly how to make a bootable USB for Catalina as I'm a bit confused as things seems to be different now ie. the apple pages say the USB drive you use should be for the very same machine you're downloading the OS to.

The only requirement for the host Mac when creating a bootable macOS USB installer is that the OS you are trying to create is compatible with the host Mac. The host Mac can currently be running any version of macOS 10.6+.

For example, here are the general requirements for creating the following bootable macOS USB installers:

• macOS 10.15 generally needs a Mac from 2012 to 2019
• macOS 10.13 generally needs a Mac from Late-2009 to mid-2018
• macOS 10.11 generally needs a Mac from 2007 to 2015

All of these versions of macOS are compatible with your 2013 iMac, so if you only have access to an older Mac, then you can start with the older OS, then once installed, just upgrade macOS to Catalina as a two step process. You can confirm OS compatibility by using the information in the following article since the model years I have listed are just generalities since there are some models which may fall outside of the ranges I have indicated:

https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

Here is an Apple article with instructions for creating a bootable macOS USB installer including links to download the various macOS installers (read the instructions carefully):

Create a bootable installer for macOS - Apple Support

ricardo bristol wrote:

It will be stick on the load screen (apple logo) and progress bar - sometimes progress will whizz up to around 90% and be stuck for hours or then re-boot. Doing the following keyboard shortcuts (in no particular order) has resulted in the following...

Sounds like you could have a failing hard drive. You need to be very careful if this is the case because the more you try to use it, the worse the failure can become where even a professional data recovery service may be unable to recover any files.

CMD-R - it occasionally tries to do a recovery. Progress bar is slow and stops just after 50%. Often reboots or goes blank after a short while - Has never worked

If the internal hard drive is failing, then this can also fail because it will try to load Recovery Mode from the internal drive. You need to try using Internet Recovery Mode using Command + Option + R to attempt to access the online Catalina installer. Unfortunately some Macs may only boot to the online installer for the version of macOS which originally shipped with the system from the factory....which is fine if you have an external USB3 drive to use for an external macOS boot drive installation that would need to be upgraded to Catalina in order to be able to attempt access to the data on the internal drive since older versions of macOS don't understand the new drive layout & APFS file system used by Catalina.

OPT-CMD-SHIFT-P-R - seems to reset PRAM and and reboots itself but again won't get past load screen.

You have mixed up these keys.

PRAM Reset is: Command + Option + P + R

I took it to Genius Bar today and they did the Apple Hardware Test (the one that looks like OS 9 / Platinum UI) and it showed that one of the SODIMMS (8GB Crucial) was bad. So we took out the bad 8GB and switched the two 4GB RAM into slots 0 and 1 but I'm still in the same situation. Apple retail employee could only suggest that the maybe a new 8GB SODIMM would make a difference

I would remove the other Crucial 8GB module and just use the two original Apple OEM memory modules. Sometimes Macs do not like mixing memory and these iMacs can be very particular with which memory slots are used when using more than the original two memory modules. I don't recall which slots the original RAM should be installed, but I do know there is an empty slot between them.

- otherwise it could be a complete fresh install of Catalina and lose everything.

That is why people need to have backups of any important data. In fact people should have frequent & regular backups of their computer and all external media (including the cloud) which contains important & unique data. Apple includes the Time Machine backup software for free to back up your computer & external USB & Thunderbolt drives, plus there are other third party options as well. With the newer Macs there are a lot more new ways to permanently lose access to the data on the internal SSD due to all the hardware, software, & security changes.

Although the only hardware fault was a bad SODIMM the Genius advisor suggested that maybe the logic board is bad. I think this is unlikely - a) no fault was detected

That is not what a passing diagnostic indicates. A passing diagnostic only means that the diagnostic could not detect a fault. It is like looking out the Window to check the weather.....if you look now it may be cloudy, but five minutes later it could be pouring rain. Diagnostics are only useful if they detect a problem.

I would dearly love some advice as to how I can either get it to boot into Safe or Recovery Mode or at least how access the files on the drive.

I would try installing macOS onto an external USB3 drive (SSD is best, but a hard drive would also work although be much slower). Try using Internet Recovery Mode (Command + Option + R), or a bootable macOS Catalina USB installer (or maybe an 10.11, or 10.13 installer if you only have access to another Mac from 2007 to 2011).

QUESTIONS:

• Can I use an external boot drive to access the internal drive

Yes.

• Can I use Recovery Mode or Safe mode by using and external bootable USB?

Yes, Safe mode would require that the exteral drive was set in Startup Disk CP first.

• Can I use an external boot drive to recover files and to put on a fresh install?

Is it even possible to recover files?

Unknown until you try.

• Is it possible to access files by removing the drive inside? A pain in the neck but I'm will to try at this point.

Possible but a nightmare chore.

• Can anyone point me as to exactly how to make a bootable USB for Catalina as I'm a bit confused as things seems to be different now ie. the apple pages say the USB drive you use should be for the very same machine you're downloading the OS to.

It has to be made on a Mac Model that can run the OS version you're building.

Software Update Failed

An error occurred downloading iOS 18

Beta 3. please know this is update for me.

Totally different problem, you're on an iPhone or iPad.