While working on my Late 2013 3 TB Fusion iMac for only 5 minutes after waking it, it started slowing down while opening an application, which made me try to reboot. The reboot failed showing the infamous STOP sign. I was running MacOs Mojave 10.14, very recently upgraded, not exactly sure when. I managed to recovery boot using my Time Machine backup disk, and went in to Disk Utility. Problem was that the SSD showed but the Macintosh HD was invisible, not listed at all, no disk of a size near 3TB at all. I then created a USB boot stick using the Mojave Installer software and booted with that. When selecting Disk Utility, Macintosh HD was shown but greyed out. I tried the REPAIR option but it came back with an error which unfortunately I did not record. I then tried the Fusion repair instructions by Apple using Terminal and the results of those actions are in the screenshots below. In short I tried to recreate the Fusion disk which did not work and came back with this error: 'Your computer must have exactly 1 solid-state and 1 rotational disk drive'. I then tried the manual commands which lead to unmounting of disk0 (ssd) and disk2 (Mac HD). But now I am unable to remount disk2, using standard commands. Are there any signs my HDD has failed, or is this caused by the OS creating problems with APFS file systems on SSDs? I've restarted with the same Mojave boot disk I created but the Macintosh HD is not shown anymore. Clearly I am not an expert, which is why I am posting here to hopefully do further troubleshooting and determine the core of the problem, and then hopefully a solution! All inputs massively appreciated. (one thing that caught my attention, the list command in Terminal lists the SSD as internal physical, but for the Macintosh HD the description shows 'synthesized'. Should this not be physical as well? Does it mean the physical disk is not found any longer ie disk failure?) Screenshots of my actions in Terminal are attached below. I should have all data in my Time Machine backup. I would be more than happy to go back to High Sierra or earlier if that gets the Machine running again..! ** edit: I've read similar threads to this which were very useful but now feel I need specific help, so I started a new thread, hope that's ok!