2015 iMac Will Not Boot or Install From Recovery

dmchord21 wrote:

So, the internal is 24GB.

I was afraid of that as I was hoping you had the (mid-2015) model with the 120GB SSD.

If I do replace HDD it appears to be a simple matter of popping the screen off, moving a speaker and harnessing an SSD where the HDD lives. 

It is not as simple as it seems. There are multiple fragile easily damaged cables & connectors that are hard to see & reach which makes it extremely easy to accidentally damage them. OWC sells upgrades for these Macs and includes video instructions showing the process (keep in mind it looks much easier in the videos than it is to do it yourself).

(I also notice 24GB internal is unformatted and not writeable. Does that matter?)

No. If you end up replacing the internal hard drive with a SATA SSD, then you can recreate the Fusion Drive setup. Or you can just erase the 24GB physical SSD if you wish to use it as a small internal data drive. Theoretically you could even make it the macOS installer drive instead of a USB stick to make it easier to reinstall Big Sur later on if needed.

Which makes me wonder, what will then be the point of the 24GB internal SSD. Pretty sure I’m not going after that thing, but I wonder if I can…turn it off? 

I would make it into a macOS Big Sur installer at some point. You cannot just turn off the internal SSD, but you can have macOS ignore it so it does not show up in the Finder.

Again, recovery defaults to trying to install Sierra, and it always fails. “An error occurred while preparing the application, try running program again.” Ad nauseam.

Yes, Apple has made a mess of things in the last several years regarding their online installers accessed through Internet Recovery Mode. It is hard to tell if it is an Apple server issue or if it is a local hardware issue when this occurs. I have seen a post which has instructions for modifying the URL of the Apple server for reinstalling macOS High Sierra....those instructions can probably be modified for the online Sierra installer as well. I'll have to look for the link again.

Actually found the link, but it is for a different issue. Still may be worth trying (I was thinking of the Fix#3 section):

https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/

Somebody on some forum suggested an incompatible OS, so I did some research and figured out how to create a bootable USB drive with Terminal on my new (to me) machine. 

The Sonoma M1 won’t even let me put anything older than Big Sur in the applications folder, so I tried with that.

If you have a 2020 M1 Mac which shipped with Big Sur from the factory, then you can use that M1 Mac to download & create a bootable Big Sur USB installer. In fact, a (Late-2015) iMac is also compatible with macOS 12.x Monterey so that is also an option. However, you cannot actually launch/run the "Install macOS Big Sur" app while booted into Sonoma (if the Big Sur installer launches automatically, just quit the app). You can, however, create a bootable macOS Big Sur USB installer using the instructions in the following Apple article:

Create a bootable installer for macOS - Apple Support

Continued...

Back on the old machine, the Big Sur installation disk showed up when starting with option key, but when I selected it, the machine went back to normal start up (with no sound) of apple logo and progress bar, and once again, a quarter of the way through the progress bar, up came the support.apple/startup thing with the do not enter sign! Lol

It didn’t even get my permission. At least the Recovery start up got that far.

Did you use the instructions in the Apple article I linked? If not, then most likely that is the problem. Most other methods of creating a bootable macOS USB installer are likely to fail.

Try using a different USB stick since the quality of USB sticks is extremely poor. Macs are also very picky about the drives used for booting.

If you are finished with the internal drives, then you could erase the internal physical SSD and use it as the destination instead of the USB stick when creating a bootable macOS Big Sur installer. Within Disk Utility you will need to click "View" and select "Show All Devices" before the physical drives appear on the left pane of Disk Utility. I would suggest trying to erase the internal hard drive as well, but most likely that will fail. If you cannot erase the internal hard drive, then we can just write zeroes to the beginning of the hard drive to destroy the partition table so that macOS is more likely to ignore the hard drive & may minimize it causing problems with macOS. To write zeroes to the beginning of the internal hard drive, you need to get the device identifier for the physical internal hard drive (likely "disk1", but check Disk Utility to confirm) and use the following Terminal commands replacing "diskX" with the correct device identifier for the internal hard drive:

diskutil  unmountDisk  diskX

dd  if=/dev/zero  of=/dev/diskX  bs=100m  count=10

After writing zeroes to the beginning of the hard drive, macOS may prompt you with a notification about an unrecognizable drive and ask if you want to erase it.....just cancel/ignore it for now.

Maybe the problem isn’t Sierra, but the method I’m using to install it? I’ve got a Sierra boot USB coming tomorrow from Amazon.

I would not trust anything like that from Amazon. Too many nefarious vendors out there on the Amazon marketplace. You must be especially careful with software since it is so easy for someone to embed malware into it. And there is no guarantee it contains the full OS or was properly created.

Then again, if it is the HDD, which you did say it sounded like, it won’t matter. (But why is Disk Utility giving it the green light?) I guess I’m just trying everything I can to avoid taking this thing apart!

Disk Utility is a very poor app. It barely functions for the most basic things. And First Aid only checks the integrity of the file system. You can have a healthy file system on a bad drive. First Aid does not check the drive's health. Well technically it has a "SMART status" indicator which is either "pass" or "fail" and only triggers in extreme failures....most drive failures never reach that point because the drive usually becomes unusable long before that point. In fact, I have only seen that SMART status failure notice twice in 20 years, but have replaced thousands of failing drives in that time.

If you do not want to take the iMac apart, then install macOS to an external USB3 SSD (a USB3 hard drive will work, but will be extremely slow...it will show if the iMac will work with the failing internal hard drive still installed though).

You can try running the Apple Diagnostics to see if any hardware issues are detected. It is rare for the diagnostics to find issues even when a hardware issue has been confirmed to exist through other methods.

I can provide other methods to check the health of the internal SSD & Hard Drive. One method require installing macOS to an external drive so you can boot into a full macOS boot drive to install a third party app. The other method involves downloading & creating a bootable Linux USB stick which has a special utility to collect the health information from the internal drives.

Most likely the hard drive portion of the Fusion Drive has failed.

What size is the internal SSD? If it is 128GB, then you can just erase the internal SSD & use it as a boot drive.

If, however, the internal SSD is smaller, then you won't be able to install macOS to it as there will not be enough room. You can instead install macOS to an external USB3 SSD.

Keep in mind though that if the internal hard drive failure is severe enough it may affect the performance even when booted to external media (or could even prevent booting from external media). It does not appear to be at that point yet, but it is something to keep in mind.