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Model A1311 / Mid 2011 / 2.5 & 2.7 GHz Core i5 or 2.8 GHz Core i7 Processor

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Prohibitory symbol on boot with new Samsung 860 Evo SSD

Hello all,

I recently bought a 500 GB Samsung 860 EVO 2.5” form factor SSD to swap it out with the primary HDD in a mid 2011 21.5” iMac. Problem is, the computer doesn’t boot (shows a prohibitory symbol) whenever the 860 is inside the chassis and connected to the motherboard through SATA, however, having an external SATA to USB enclosure, I managed to install macOS anyways, and the iMac boots just fine from the SSD when it’s connected trough USB.

TLDR: I installed macOS on a SSD, my iMac boots fine from it when connected through an external USB to SATA case, but it shows a prohibitory symbol instead when the same disk is mounted inside the chassis and connected directly through the old HDD’s SATA cable.

Is there a problem with the cable? Seems doubtful, as the computer has been working fine, albeit as slow as ever, since I reinstalled the old HDD. Any ideas as to what is happening?

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First thought is the SSds format. While it will boot from any type of format externally, it will not boot internally unless it has been formatted for Mac. Usually this is GUID, Mac OS extended journaled formatted.

Did you remember to install the OWC Thermal sensor for the 2011 to prevent run away fan?

Also, what system are you trying to run?

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Hello, thank you for taking the time! No, I took care to format the new SSD to GUID/Mac OS Extended Journaled before installing macOS on it. I had to do the latter through the external enclosure, as it wouldn't finish the installation process if the disk was connected internally through SATA. I currently have 10.12.6 Sierra on both the old HDD and the new SSD.

Concerning the thermal sensor, no, I didn't buy it, I plan to temporarily run Fan Control as I initially did on my late 2009 iMac – on which I also installed an identical 860 EVO this summer with no hiccups – and to get one later on, hopefully in a couple of months.


Sometimes going into system preferences and selecting the startup drive works, you will need to unlock it with your standard password, then select the drive. Also try starting up holding down the Option key and then select your startup drive.


I have exactaly the same problem !!!!!


Except that my computer turns on and gets me to the desktop but cannot open any application or acces any web related pages when mounted internally .


tiago try starting in Safe Mode by holding down the Shift key on start up to see if you have a software issue


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