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Model A1311 / Late 2011 / 3.1 GHz Core i3 processor / Education only model

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Why will an iMac 21.5" Late 2011 not accept an operating system?

I am working on a Late 2011 21.5” iMac and went to install a new Solid State Drive into the computer. After attempting to install High Sierra with the new SSD as the internal drive (and having it fail), I installed High Sierra onto the SSD externally on a separate computer and was able to successfully install the operating system onto the SSD. However when I reinstalled the SSD into the iMac, the computer did not accept the already installed operating system.

I have since installed the same SSD with the same operating system I had installed onto it into another iMac Late 2011 21.5” and that computer loads up without any issues.

Is this a hardware issue inside of the iMac or a possible firmware lock?

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Let’s do this!

From your working system connect the SSD externally using a SATA to USB adapter (or drive case via USB or TB) Reformat it to GUID Journaled file system (HFS+). Now install the SSD into the system in question. Now using a bootable USB OS installer with Sierra Not High Sierra! Install Sierra onto your system. It will update the systems firmware. Now if you really need High Sierra install that afterwards!

OK, why?? Your systems firmware was still running an older version that can’t be upgraded to High Sierra until you get the Sierra. This gets into the fact HS alters the file system of SSD’s to APFS (if you had installed a HDD the file system would have stayed at HFS+). So when you prepped the drive externally or in the other system the firmware was no updated with the version that understands APFS!

Now the other shoe drops! SATA based drives don’t run well with APFS! Without getting deep here. APFS is a very chatty dialog unlike what HFS+ uses. It was designed with PCIe/NVMe drives which have a wider I/O and many more buffers.

Let’s think this like an old fashioned fire brigade using buckets of water passed between people in a line. One line would pass the full buckets from thew ell to the fire and the buckets would be handed back so they are constantly crossing each other one full for one empty. How fast can you go? Likely a few buckets will be dropped both full and empty! Not very good ;-{ Now what if we had a second line of people who only handled the empty buckets then we would remove the conflict of the people dropping buckets! SATA is just like this one line which needs to handle both the movement and the Ack of the movement, so if we have a chatty dialog we get into the higher flows of Moves and Acks!

Of course we could even add more lines (PCIe lanes) and as PCIe has many more buffers can queue up things much better!

So bottomline: SATA based systems just work better with HFS+ on either HDD or SSD drives!

I do I like APFS! But on PCIe based systems, I would only use the Mojave version not High Sierra as Apple altered APFS between them.

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Now to add a bit more here Apple messed us up! Here's the dope on that If you've got an old macOS install image, it will probably stop working today

Here's the fixed OS installers:

How to upgrade to macOS Sierra Jump down to Step 4 click on Download macOS Sierra to get Sierra.

How to upgrade to macOS High Sierra and again, jump down to Step 4 click on Download macOS High Sierra to get High Sierra.

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