Unable to access startup security utility

siddonair wrote:

I included the last two pictures as there was nowhere in Base OS X that I could find that would allow me to boot from a USB, as the drive was not showing up in the boot options but was showing in the disk utility.

This means the physical drive is seen as shown in the first picture. The fact you don't see any bootable volumes on the Apple boot picker menu when Option Booting means there is no bootable OS on that drive even though it appears there is some sort of residual or partial copy of the macOS installer on the internal drive.

My guess is that the internal hard drive is failing.

This is the only Apple device I have, so I used Transmac to create a High Sierra installer on a USB drive.

While that may have worked years ago, I have not seen any recent success stories with that method.

I have tried to boot into the network installer using Cmd+Opt+R but it does not seem to do anything different to a normal boot without pressing any keys.

It should try booting to the online installer. So when you use Command + Option + R you are not even able to access the online High Sierra installer as shown in your first picture? How did you boot for that first picture?

If you are connected using an Ethernet cable or your WiFi credentials have been stored in the NVRAM (aka PRAM) then you may not be prompted for authenticating to the WiFi. You should be able to boot to Internet Recovery Mode or at least get some kind of error message if it fails. Are you connecting with an Ethernet Cable or over WiFi?

Did you try using a wired USB keyboard to make sure the key sequence is accepted? Sometimes a wireless keyboard may not work. Also the timing to press those keys can be critical. It is best to press them immediately after hearing the startup chime. Holding them before the startup chime may cause the keys not be registered. Pressing them too late will result in the same.

What version of macOS was installed on this computer previously?

Would this mean that the only option I have left is to remove the hard drive and transfer in the OS installer files externally?

Except you don't have the installer files or at least ones we can rely on and you would still need access to another Mac to do so anyway. Unfortunately Apple makes acquiring macOS installers & creating bootable maOS USB installers very difficult. Apple assumes everyone has a working Mac, or if you have a broken Mac that you have access to another compatible Mac to download & create a bootable USB installer.

Try an SMC Reset and a PRAM Reset (hold the PRAM Reset for at least three chimes) to see if that makes any difference booting into Internet Recovery Mode using Command + Option + R.

At this point your only boot options are to try to access the macOS 12.x online installer through Internet Recovery Mode (Command + Option + R) or by using a bootable macOS USB installer created using the official Apple instructions. Anything else will just lead to more confusion & frustration. If you don't know anyone else with a compatible Mac, then see if an Apple Store or an Apple Authorized Service Provider can assist you in creating a bootable macOS USB installer.

What was going on with this iMac prior to losing the OS on the drive? That may be relevant as well to the current situation.

The second two pictures have nothing to do with booting from USB other than to block booting from anything but the internal drive without providing the firmware password. The Startup Security Utility is a feature of the 2018+ Macs.

Unfortunately it seems Apple has some issues with their older macOS online installers such as High Sierra. Have you tried booting into Internet Recovery Mode using Command + Option + R to attempt to access the latest online macOS installer compatible with your Mac (for a 2014 it would be Monterey). Unfortunately sometimes no matter the keys used to access Internet Recovery Mode, it may still only boot to the online installer for the OS which shipped with the Mac from the factory.

When making a bootable macOS USB installer, you must acquire the macOS installer from Apple (using a Mac compatible with that OS) and use the instructions in this Apple article to create the bootable macOS USB installer (also has a link to download the installers as well):

Create a bootable installer for macOS - Apple Support

If you don't know the exact model of your Mac, then enter the system serial number here:

Check Your Service and Support Coverage - Apple Support

siddonair wrote:

When I start the mac up without holding any keys, it boots to a menu with four options: OS install, time machine, disk utility and get help. Clicking on the OS install option, it takes me to a page that says to install high sierra, but after clicking continue I am met with the error saying that the device is unable to connect to the server. I have made sure that my internet connection works, and this problem even persists through ethernet, so I assume Apple have dropped support for their High Sierra network installer.

The fact it boots automatically into recovery mode is a sign that either there is no bootable OS on the internal drive, or it confirms a hard drive failure.

Yes, the online High Sierra installer is known to have this issue....so far Apple has not fixed the issue. That is why I wanted you to try booting the iMac using a wired USB keyboard in order to enter Internet Recovery Mode using the keys Command + Option + R to attempt to access a higher version of macOS installer which doesn't have this problem. The timing of pressing & holding these keys is critical...best to press them immediately after hearing the startup chime (if you wait more than a second or two after hearing the chime, it may be too late with some later models since they boot so quickly). It is important to use a wired USB keyboard as well because sometimes a wireless keyboard may not be connected & responding soon enough to register those keys. Of course some Macs will boot to the older online installer no matter the keys used for booting.

With what you have said about the unreliability of transmac, I think my best course of action would be to get an installer from a friend with a mac and see if that makes any difference.

You will need to get the exact model of your iMac to know exactly which versions of macOS are compatible with it. Make sure to use the Apple "check coverage" link I provided in an earlier post to identify the exact model of your Mac.

When you have the exact model of your Mac, then you can use the information in the following article to figure out which versions of macOS are compatible with your iMac. You can also use that article to make sure your friend's Mac is also compatible with at least one OS which your iMac can use. If so, then you can create a bootable macOS USB installer for that particular OS.

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

Regarding the previous use of the mac, I have no information at all. It was found in an attic and i do not know any of the previous system details.

If the hard drive is failing, I think I am just going to call it quits with the mac. I am not fond of the idea of tearing down an iMac in the slightest

If the internal hard drive is failing as I suspect, you may be able to use an external USB3 SSD with this iMac as long as it is a 2012 or later model which has a USB 3 ports (any older Mac would have USB2 ports which would be too slow for booting an external SSD although it technically would work). You can install macOS to the external SSD and boot from the external SSD...it will likely be faster than booting to the internal hard drive (hard drives are very slow...at best 100MB/s, but usually slower....whereas an external USB3 SSD should provide you with about 400-500MB/s speeds which is as fast as if you replaced the internal hard drive with an SSD upgrade). However, if a hard drive failure becomes more severe, it can affect the system performance even when booting to an external drive....that doesn't seem to be the case here since you are able to boot into recovery mode without issue.

Good luck.