Mac won't boot or go to recovery

from the GSmartControl log:

ID# ATTRIBUTE_NAME VALUE WORST THRESH FAIL RAW_VALUE

5 Reallocated_Sector_Ct 100 100 005 - 187

9 Power_On_Hours 036 036 000 - 28190

193 Load_Cycle_Count 020 020 000 - 800175

196 Reallocated_Event_Count 100 100 000 - 237

197 Current_Pending_Sector 097 097 000 - 280

Your hard drive is failing. Even if you didn't have Reallocated Sectors/Events and more sectors waiting to be reallocated, the drive is almost worn out anyway as evidenced by the "Load Cycle Count".

This DriveDX article has information on how to interpret these SMART attributes (aka "Health Indicators") if anyone is interested. While the information is good, it doesn't necessarily apply to each attribute fro every drive.

https://binaryfruit.com/drivedx/help#ui-drive-health-dashboard-important-health-indicators

(version of knoppix is in German and it is taking a while to navigate).

There are two versions of Knoppix available to download. One in English where the file name includes "EN" and one in German identified by "DE". IIRC you can choose the language when booting Knoppix on a regular non-Apple PD, but the Macs don't allow Knoppix to display the early boot menu where you can choose the language.

One thing I have noticed - I was having problems with the system continually hanging, and was going to install a clean mojave assuming some sort of hard to track down conflict was happening. Knoppix seems to be hanging regularly for 10-20 seconds too. Surprised at this, given the very clean installation...

This sometimes happens when get a lot of bad sectors on a hard drive. This also means you won't be able to run the iMac booted from an external drive so your only option is to replace the failing hard drive. I would recommend upgrading to an SSD as it will give your iMac a very nice performance boost, but you will most likely need to also install a temperature sensor adapter cable so the iMacs fans won't run at full speed with a third party drive installed.

Replacing a drive in an iMac is not an easy task so unless you are very skilled at working on computers I strongly suggest you have an independent Apple Authorized Service Provider perform the SSD upgrade for you.

It sounds like a failing hard drive. The easiest way to check the hard drive is by creating a bootable Knoppix Linux USB drive using Etcher (Mac/Windows/Linux). Option Boot the Knoppix drive and select the orange icon labeled "EFI". Once at the Knoppix desktop, click on the "Start" menu in the lower left of the Taskbar and navigate to "System Tools --> GSmartControl". Within the GSmartControl app double-click on the iMac's drive which will open up a window containing health information about the drive. Post the report here using the "Additional Text" icon which looks like a piece of paper. Post the report for each drive in your iMac if you have a Fusion Drive setup.

Edit: Give Knoppix time to boot. If the hard drive is failing, it can really affect the boot times from other media, plus some USB drives can be extremely slow. Sometimes a Mac may appear frozen with no mouse movement and it may not appear to be booting Knoppix due to the peculiar nature of the Mac boot process. Give it some time and you may see Knoppix boot. It is rare Knoppix doesn't boot on a Mac.

It sounds like you have the necessary skills, but it can still be difficult since the cables inside are very hard to access since they are so short while holding the LCD panel. The cables and connectors are very fragile. Definitely review the iFixIt.com repair guides and maybe a YouTube video. When removing the Display keep it upright. If you try to lay the LCD flat you risk it cracking under its own weight.

You're most welcome. I'm glad it was easier than you expected. It sure would be nice if those cables were just a wee bit longer or placed just a tad bit higher.

Congratulations on the upgrade and enjoy your new SSD. It should seem like a new computer.

I'd say HDD, but also with the age might also have something with the motherboard.