Late 2015 iMac (17,1) w/ Fusion Drive Unexpectedly Restarting (Kernel Panics)

Hey there!

You're correct! It does appear that the Fusion Drive has become split. However, before creating a Fusion Drive again, I'd recommend you check the health of the disks. If one is failing, that could be the reason for the unexpected restarts.

To do this, I recommend you download and run DriveDx. It can tell you the health of the hard disk and solid state drive.

-Jack

The I/O errors on the SSD are concerning. You can try to create the Fusion Drive again by following these steps: How to fix a split Fusion Drive - Apple Support. If you attempt to fix the split Fusion Drive, be sure to backup your Mac first: Back up your Mac with Time Machine - Apple Support.

If the issue persists, or if you'd prefer to bypass the internal Fusion Drive, I'd recommend you setup an external SSD as your startup disk. This user tip should help with that: Use an external SSD as your startup disk … - Apple Community. If the SSD I recommend in the user tip appears out-of-stock at the Apple Store, the OWC Envoy Pro FX - OWC is another great option.

-Jack

It actually now appears that the SSD has completely disappeared. So, there still isn’t a Fusion Drive, only a hard disk. If you have performance or stability issues, I’d strongly recommend you look into the external SSD method I linked above. It should be a far more reliable (and faster) setup than what you currently have. 

-Jack

Before you do ANYTHING and I mean anything you must Backup your computer. There is no evidence that you do which means all of the data stored on it is at risk. Operating any computer without daily backups is a 100% guarantee of lost data. Please get a new External Hard Disk and let Time Machine backup the computer!!!!!!!!

I just realized that you got the computer used from your friend. Before anything is done, it must be setup per the directions in What to do before you sell, give away, or trade in your Mac. Most people that get used Macs from unauthorized (friends, e-bay, craigslist, marketplace, local computer store, etc..) do not have the machines setup for you in advance so it is left to you to do.

If you ignore or do not follow the directions your experience will continue to be very poor and the computer will not run correctly. If you need to, you may have to have your friends assistance if his passwords are required.

In retrospect, you also may just want to consider taking the computer to your local third party (non Apple Store) Apple Authorized Service Provider who can help you as they have the knowledge and proper tools to work on the system. We see do-it-yourselfers all the time that in their attempt to save money either end up spending more money or simply wasting their time and efforts. My suspicion is you don't have much experience working on Macs or Mac OS so it may be in your best interests to take this step. If you don't know of any third party AASPs in your area, please click Apple Authorized Service Provider Locator (AASP) & Genius Appointment or you can call AppleCare and they can help you find one make an appointment for you. AppleCare Contact Info

Good luck!

Would a factory reset repair the Fusion assuming it's not a hardware issue?

Yes, it would. It would also repair the First Aid error, assuming it's not a hardware issue. Also, for the other external SSD option I linked above, I made an error. Instead of the Envoy Pro FX, for your Mac, you'd need the OWC Envoy Pro Elektron - OWC.

-Jack