SSD issue

I got a used Dell r710 the day. I decided to get one of them fake DVD drives that have the Sata port inside. I bought a 1 TB SSD for the OS of proxmox. If I  hook up the SSD via an external USB adaptor its works but if I use the fake DVD adaptor it will not boot but I see it in the BIOS plugged into SATA 1 port and show what type of SSD it is. Any ideas why I can't boot from it but the BIOS see's it?