Chosen Solution
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?
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? https://eshop.macsales.com/item/OWC/DIDI… Also, what system are you trying to run?
