Secondary hard disk prevents me from booting on my SSD.

[mb1601]

Currently running Mojave and everything works correctly.
Except whenever i connect my second hard disk via SATA cable, it makes it unable to boot my SSD (or any other thing).

Behaviour: When booting, with the second hard disk connected and i try to boot from my SSD, it sends me straight to BIOS.
And sometimes it reads "boot0ss: error".

NOTE: This is only when the secondary hard drive is connected.

EDIT: I'm also running this build without a GPU for now. Planning on purchasing RX 580.

 

[blurble]

Did you set the boot order in BIOS to boot SSD first? Maybe obvious but not mentioned so...