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The hard drive (a Fusion Drive) on my 2015 iMac has become badly corrupted, possibly getting into that state via some incompatible ram that had previous been installed in it. (The ram was removed and the computer is in its stock configuration now.) The computer won't boot past the login screen or boot into recovery mode without a kernel panic.

When I took it into have it examined by an authorized repair shop, the tech told me that because the kernel panic prevented booting into recovery mode or target disk mode, that the hard drive needed to be replaced. When I asked whether the drive could be wiped, she said no and that it was impossible without cracking open the computer.

Is this true? In a standard Windows/Linux based Intel architecture PC, any drive can simply be ignored at the bios level. Does corruption of the internal hard drive in an iMac really prevent any possible boot attempt via an external drive?

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    You said, "When I asked whether the drive could be wiped, she said no and that it was impossible without cracking open the computer." And to that I'd say, unless she already tried, she doesn't know what she's talking about. You can boot the computer from a macOS USB Installer and wipe the drive and reinstall macOS. (That's assuming there is nothing physically wrong.) – user3439894 Aug 28 '18 at 18:34
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    Thanks. I figured out a full solution and posted it here. It's strange behavior that the Recovery Mode touched the disk as it booted and induced a kernel panic. But from that she somehow deduced that meant a physical disk problem in need of replacement. It doesn't follow. I think she was just being lazy. – WildGunman Aug 29 '18 at 3:53
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    Back in my Windows days, I used Knoppix Live CD when first available over 15 years ago to test PC hardware independent of Windows being installed to see if it was a hardware/software issue. Have used may different Linux Live Distros on CD/DVD/USB since then and nowadays always have a Linux Live USB handy. :) Glad you were able to resolve without needlessly replacing the HDD/SSD. – user3439894 Aug 29 '18 at 4:14
  • I remember when the Knoppix Live CD image first came out. It was a revelation. A complete noob like myself could query the entire machine without even touching the installed OS. It was all just there, in beautiful GUI, with the touch of a button. – WildGunman Aug 29 '18 at 13:33
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No, corruption like what you described here (i.e. non-hardware failure of the disk drive) does not prevent boot attempts on an external drive.

In fact you do not even need an external drive. You can just start up the iMac in Internet Recovery Mode to reformat your internal disk and reinstall macOS. You do that by holding down Command+Option+R when powering on your iMac (hold it until you see a spinning globe saying Starting Internet Recovery).

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For whatever reason, Internet Recovery Mode still tried to query a part of the original disk that was corrupted and induced a kernel panic. Booting up into a macOS USB stick created the same kernel panic, again at boot, not when trying to do an install. 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.

For the life of me, I sometimes wonder why I bother going to these repair places. They never know anything and then when you confront them with questions they just lie to you.

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