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Model A1312 / Mid 2011 / 2.7 & 3.1 GHz Core i5 or 3.4 GHz Core i7 Processor, ID iMac12,2

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'Mac OS cannot be installed on this computer' recurring error

So, my mid-2011 iMac suddenly became ‘obsolete’ by Apple standards in Jan 2019 and funnily enough a few months later I start having my first issues with the system in 7 years.

I’m running on High Sierra and have been for a while, yet upon booting up my system one day was met with a long load screen and a message saying my Mac OS can’t be installed.

I reinstalled High Sierra which seemed to work but a few days later was met with the same issue. Even when the system is working my boot up times are noticeably slower.

Does anyone know some other possible fixes?

Answer this question I have this problem too

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How big is your boot drive and how full is it?

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Boot drive is a good 900+ GBs with at least 40 GB free.

Another development - I have two internals HDs in my Mac. I switched to my second to check on it then when i tried to select my original HD to return it as my start up disk (the one I'm having problems with) - its no longer visible.

Any ideas why it may have disappeared?

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OK, it’s starting to should like you have a failing drive. Lets run Disk Utilities First Aid on the drive, did it pass without error?

Can you give us the specs of the drives Make & Models

Update (04/20/2019)

OK, If the drives passed Disk Utilities First Aid then I think your issue is a mis-match with your OS. You see High Sierra is the highest version your system can support. It appears some how your drive is being upgraded to Mojave so when you boot your system up its reporting the OS (Mojave) is not supported on your system which is technically correct from Apples perspective!

Did you click on the Mojave upgrade some how?

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Before the drive dissappeared from my disk utility view I did run first aid on it and it passed without error.

The HDs are SATA Internal Physical Volumes.

Is that right? Apologies if not, my knowledge is novice at best.

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Yes the 2011 iMac's use SATA based drives.

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monaco19 will be eternally grateful.
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