Reporting back - internal SSD swap appears to have addressed the kernel panic reboot issue, also blowing out almost 10 years of dust. Maybe high temps were the culprit. Like many others I seem to have damaged my screen, now have a vertical line, but it's not enough to warrant a replacement of the computer. I have a functional iMac that is running way faster for a small investment compared to what I was looking at, plus a bunch of my own time trying to sort this out. Wish I would have done this a few years ago prior to drive failure. Some notes: 1. copied all contents of HDD to backup external SDD when things started looking bad a few months ago to be safe and made that bootable. I could have done this with the replacement 2.5 inch SATA SSD but I didn't know I needed this prior to backing up. 2. erased all data and unmounted the internal Mac SSD of the Fusion Drive. The HDD was already dead at this point with no data. 3. Ordered the SSD replacement kit from ifixit, followed the steps very closely also watched some YouTube videos. be very careful when cutting adhesive, and disconnecting and reattaching the power supply cables from the display the instructions are ambiguous and you can damage these and kill your Mac if you work too fast. watch a couple videos on this before attempting. Once the display is off if you work slowly and take your time this is pretty straightforward. Do NOT close your screen, up leave it propped open and don't remove the second set of adhesive tabs until you are sure everything is working fine and you have a bootable SSD. I wasn't entirely clear on how to set up the SDD before I installed it so I just left it unformatted. I booted from my external USB drive, then used disk utility to initialize the new drive. It took some fussing I probably messed this up a couple times. Once I got the drive formatted I couldn't figure out how to get disk utility to restore from the USB drive so I took it back out f of my Mac, put it in the external USB enclosure ifixit sent me, attached it to my M1 laptop via a hub, used carbon copy to clone my external SSD that had two volumes - one a bootable startup and one for data. I thought that would allow me to just load the SSD and boot from that but no. I had to use the disk recovery to load a fresh copy of the OS onto the SSD to do that. I had to also create a " Base system" volume for the new SSD and restored that from the external SSD base volume and loaded OS to new base system volume. After rebooting I went into to disk recovery and changed my startup disk to my internal SSD base volume, rebooted, and everything seems to be working ok. My wife is annoyed at how long this took. Pros could probably handle this a lot more efficiently. Note that an obvious reason to replace and old iMac w a silicon Mac is data transfer speeds are order of magnitude less on these old machines so moving data using USB A cables takes forever.Once you bite the bullet on moving to a new machine it should be a lot less painful to move stuff to external disks and new machines than working with these old beasts. Having another computer with a hub was necessary for me to swap between my Samsung SSD and Crucial SSD. Looking back on this, If you are sure you need to swap get the internal SSD w an external USB enclosure, clone your HDD to that if its still alive, test it by booting your existing machine from your new SSD inside the USB enclosure, if its all good, then pop that sucker back in your mac and you should be good. I hope this thing survives until the m4 line comes out.I will report back if the iMac starts acting up again. Good luck!!!
