Sierra build doesn't recognize boot drive anymore

[Nyak]

I've had Sierra running for three weeks now and it's been working perfectly. But today I turned on my computer to find it booted into the BIOS rather than Clover and my SSD wasn't on the boot list. I tried switching out the SATA cable and it booted successfully once, but the problem has since resumed. Where do I start troubleshooting?

 

[Azial]

In the BIOS, you can set the boot priority / order. The default order is determined by the SATA plug number on the Mobo. I therefore suggest plugging the system drive to number 0.