Why do I have to manually select a bootable disk in the BIOS?

Question

Windows

Good day, the problem is that after turning off / restarting the PC, you constantly have to press DEL and then F10 (Save and exit) to start the system, or F12 and select the disk on which Windows is installed. If such operations are not carried out, then a black screen appears with the text: "Select the first disk" or something similar.
If it is important, then: I took out the battery from the mother.
Mother: gigabyte z370p d3.

Answer 1

The battery is dead or incorrectly inserted - these are the settings and reset to default.

Answer 2

Good afternoon!
It looks like the partition flags are not correctly set. and the "Bootable" flag is not selected for the OS disk. The easiest way is to boot from different (available in the open) boot resuscitators and with the help of Acronis, PartitionGuru, PartitionWizard, etc. check flags.
As an option, you can also use them from Windows.

Answer 3

It seems to have been decided.
I took the battery out again and put it back in. I went into the BIOS and reset all the settings in general, reconfigured the start order of the disks (I have 2 of them) and it seems like everything started up the first time.