Why does my iMac crash and restart after cursor disappears?

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

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The internal HD is failing so to be blunt, it's dying. I see you use Backblaze to backup, I'd recommend backing up to a Time Machine External Hard Disk too if you do not already.

You can have the computer serviced and have the HD replaced however considering this is a base model 21.5" iMac if it were mine I would seriously consider replacing it with a new 24" iMac. You do have another option which is to get yourself an external SSD and use that as your startup drive. That will cost about $100-200 and make the computer considerably quicker. Perhaps your local Apple Authorized Service Provider can help you get that done if you do not want to replace the HD.

Post the entire Etrecheck report according to the following:

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Then we can evaluate the report to see if we can determine the cause of the problem.

I suspect Mac OS may be damaged or have a corrupt area. Please restart in the Recovery Partition (hold down Command + R on startup) and then install Mac OS on the internal HD. This will not affect your data, however before you do this you MUST backup the computer using Time Machine, I see there is no evidence you backup the computer which is shortsighted and a 100% guarantee you will lose data. If you have never heard of Time Machine please refer to Back up your Mac with Time Machine. You will need an external HD to backup to, I'd recommend a OWC Mercury Elite Pro a 2-4TB model is fine.

I don't see any of the usual suspects. However, the Mac you purchased was the slowest Mac offered by Apple at the time: 8 GB RAM and a 5400 rpm hard drive.

Your read and write speeds are lower than they should be:

 Write speed: 86 MB/s

 Read speed: 87 MB/s

Notes is taking up a lot of system resources. Make sure you have the latest erosion installed for you system.

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac andreboot normally and test to see if the problem persists..

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Download the free version of DriveDX and run it on your internal drive. If ANY errors appear, then your HD is failing.