Prohibitory symbol on boot with new Samsung 860 Evo SSD in 21.5" iMac Mid 2011

[syntholog]

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 sign) 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?

Sincere thanks to anyone who'll take the time to answer me

 

[Juicy Box]

What OS is on the SSD? What OS did you have installed on the OEM drive?

I ran into a similar issue,the SSD worked externally, but wouldn’t install the macOS 10.13 internally.

it ended up being that firmware needed to installed for the SSD to fully work, but the firmware would only install on an Apple OEM drive.

if I would have installed 10.13 prior to swapping the drive, this wouldn’t have been an issue, because the firmware would have been updated then.

I reinstalled the iMac’s original HDD, which I was a little concerned that it could fail at any moment.

I updated the MacOS to 10.13, the firmware was updated then, and I swapped the drive again for the SSD. Everything installed fine then.

Another issue that I had, was that the SSD wasn’t fully seated in the Icy Dock adapter I was using.

 

[syntholog]

Hello, thank you for taking the time to answer! Both the HDD and the SSD have 10.12.6 installed. Should I try updating to 10.13? What kind of firmware are we talking about here? Thanks again!