@justinobwi I believe the only currently supported boot configurations are via microSD or the internal USB-A port (with a number of posts discussing booting from various M.2 adapters):
You actually may be able to boot with only a single SSD or hard drive connected to one of the SATA connectors (with nothing connected to the other 3 SATA connectors or the internal USB-A port) but that would be wasting quite a bit of the EON’s capabilities.