Is there an SSD onboard plus socket

Replaced SSD in my HBPR . Boots into old system as if SSD not replaced. Are there two SSDs? one builtin and an additional socket? Can both be active as separate devices? A BIOS setting? TIA George spix

I should have included that the replacement SSD was a new SSD, right out of the box. I made no attempt to image the existing SSD onto it ijust wanted to replace the old and install another system from scratch. George