iMac mid-2011 completely freezes few mins after startup

Your SSD has been running hot and has reached 70C. When SSDs reach 70C the SSD speed is throttled in order to lower the temperature of the SSD, but the report shows the time in an "Over-Temperature" state was 37 minutes (not sure if this refers to a single event or if it is counting multiple events -- I'm guessing the former which could easily explain the SSD's disappearance). Since you had to manually set the fan speed it may indicate your iMac's fan is not properly adjusting its speed. I believe the 2011 iMac needs a special temperature sensor adapter when using a third party internal drive.

Try running the Apple Diagnostics to see if any hardware issues are detected with the fan or the temperature sensors. It may give an alert about the SSD, but that may just be due to being a third party drive. The rest of the DriveDx report looks Ok.

It is hard to say whether this SSD is failing or whether you have another issue with the iMac (fan, or sensor) which is causing the issue with the SSD. I guess you could try running the iMac from an external macOS boot drive to see how the iMac works. If it works fine while booted externally, then most likely the internal SSD is bad. If you have problems while booted to the external drive, then you likely have other hardware issues with the iMac.

oersadam wrote:

Remark: as the computer freezes so fast, the only way to run and generate the report was to manually set the fan speed to 5K RPM using the SSD Fan Control app. I have to use this app, as my computer was retrofitted with an SSD and the system could not control the fan (it was always running at very high speed).

Before the freezing what speed did you have to run the fans so you could operate?

I don't see anything in the report that would be an indication of freezing of the iMac. The problem is that the SSD was not installed with the correct cabling which would include a temperature monitor so that the system could control the fans.

What I see is that you don't have enough RAM to adequately handle the load being asked of it: 4 GB.

Seeing that it's a 10 year old model and is considered obsolete by Apple I don't see much hope for it. I'd get a new Mac and remove the SSD to recover your files (unless you have a current backup of the data).

If you decide to go with a new Mac I'd wait until next Monday's Apple Event where there will be some announcements of new products. Hopefully they may be announcing the new 27" iMacs. Otherwise I'd suggest you go for the current M1 24" iMac with 16 GB of RAM.

It may be an issue with the SSD since the Kernel Panic listed mentions "not being able to recover the SATA HDD".

Run DriveDx so we can check the health of the SSD. Post the complete health report here using the "Additional Text" icon which looks like a piece of paper. I'm not sure anything will show up in the SSD's health report since the SSD is going offline, but it is worth checking just in case.