Samsung 860 Evo 500GB not detected

I would get a SATA to USB adapter cable to see if the drive is accessible externally, if it is then you know the internal data cable is bad and will need to be replaced.

The Samsung 860 EVO drive has no issues running in SATA II (3.0 Gb/s) systems like yours.

But, you maybe facing another issue here! The older installers have a certificate which may have expired on you. Get a fresh one from here How to upgrade to OS X El Capitan jump down to Step 4 locate the blue URL link Download OS X El Capitan to get the fresh installer.

Here’s a bit more on the certificate issue If you've got an old macOS install image, it will probably stop working today