Prohibitory symbol on boot with new Samsung 860 Evo SSD
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?
Is this a good question?