Cost for replacing a hard drive on a 2015 iMac (5K, Fusion Drive)

Not to be snarky here, but this was an "Apple Authorized Service Provider!!!" whom I scheduled using the Apple website. They were either incompetent, scammy, or both.

I don't fully understand the nature of the OSX recovery boot scheme, but attempts to boot to Internet Recovery Mode or to an OSX USB boot stick try to query a part of the original disk that was corrupted and induced a kernel panic. However, I was able to boot up using a USB Ubuntu live distro, which worked fine. From there I wiped the disk. Then Internet Recovery Mode worked fine, and I re-partitioned the disk and restored from backup no problem.

Long story short, deep level disk corruption can apparently sometimes interfere with the normal mode of operations that macOS uses to boot from recovery media. A non-mac operating system will ignore the disk entirely and boot the machine just fine. From there, the disks can be accessed as simple attached devices. Then a completely wiped disk will be ignored by recovery mode without issue.

It appears as though the hard drive is corrupted on some deep fundamental level. As far as I can tell cloud recovery mode still touches something on the hard drive that induces a kernel panic before it can attempt to remotely boot the OS. The same goes for Target Disk Mode with a USB installer. I asked tech support and they told me there's nothing for it except to take it in, since the drive cannot be manually wiped without cracking the computer open.

Taking the computer into the Apple Store for repair is a waking nightmare, and they never tell me anything I don't already know. I just want to know roughly what its going to cost before I do it.

I will try. I actually need to re-confirm some of this myself. I took it to an authorized Apple repair shop, and the tech ran the basic diagnostic set and swore that the hard disk was just "bad" because it would panic before initiating recovery mode or target disk mode every time it "touched the hard drive". She was so ready to jump the gun and so oddly un-knowledgable about the boot process that I just asked to take the computer back.

I'm going to try it again when I get home later today. As I mentioned its possible that she's just incompetent or she's lying to me.

Here's a technical question that you can perhaps answer for me. To what extent does an iMac need to interface with the original existing internal hard disk in order to boot? I know in a typical Windows/Linux oriented Intel architecture, the system is fully capable of just ignoring certain internal core disks at the bios level. I had assumed this was the case here. Am I wrong?

No, it fails outright. It will kernel panic immediately out of target disk mode after selecting a bootable USB. Attempting internet recovery via Option-Command-R will connect to the internet, begin the initial download process, complete, drop into the "black on white Apple" boot screen and then kernel panic again.

Whatever is going on, the normal built in subroutines for coaxing the system back out of its failstate do not work.

This is what I thought, but the Apple tech insisted otherwise. It‘s admittedly weird that Apple’s own external boot proceedure would be interfered with by the software state of the existing hard disk, but it doesn’t logically follow that the disk necessarily had a bad physical sector. And to be clear, the tech basically told me that remote wiping the disk was not possible.