I have an AB350 Pro 4 from Asrock, Bios is 5.10. 

No matter what i do in Bios it does not save the correct boot option.

I checked the sata cables and cables from PSU, everything is in place.

My SSD is the main partition with windows, my hdd ist just for storage no windows installed there.

When i Change my hdd in the UEFI-option , and change it for the windows ssd it does work.

After a restart it does not work anymore and it selects the HDD.

The error i get is no boot device found. After a restart it finds the ssd and boots into windows.

I checked my windows and my drives for errors and could not find anything.

The SSD is on 99% on crystaldisk, the HDD checked with the Toshiba tool and in crystal disk shows no errors.

Windows 10  shows me the "boot sector" only on the SSD.

Specs:

CPU: R5 1600 non OC

MOBO: Asrock AB350 Pro4

RAM: Corsair Vengeance CL16 3000Mhz

GPU: Gainwaird Phoenix 1070ti

SSD: Crucial MX 100 256GB

HDD: Toshiba HDD 1GB HDWD110

PSU: BeQuiet Straight Power 500W.

Case: Fractal Design R5

OS: WIN10

Edit:

 I forgot to write that i bought this board 1 year ago and did not have this issue before. The first time in installed Windows i had a very slow boot on windows that i fixed after i turned off FAST BOOT in Windows 10.

I changed my Case 2 Months ago, but the boot order error did not occur then. After 1 week  i reinstalled Windows 10 because i upgraded my GPU.