Dual Boot Only Boots Into Default OS- "Choose Operating System" broken

I have a dual boot system, and at the blue selection screen "Choose Operating System", it offers the choice between two builds of Win10.

Before an update 'broke' it, I would just click on which OS I wanted, and it would boot into that OS. Now, it only allows me to boot into the *default* OS, even though it still offers me two choices at startup.

If I click on the non-default OS, the computer reboots and then hangs at the splash screen.

I can still access the other OS, but to do so, I have to power off, reboot, and change the default OS in the options to make it work.

I want to restore the ability to choose which OS I boot into by simply clicking on it. Thanks in advance for your help.

Note: I don't know if this is important, but this laptop has a 250GB PCI Express 3 M.2 Solid State drive, split into 2 partitions, which contain each OS.

What happens if you turn off fast-startup?

Try running powercfg -h off from an administrator command prompt (on both instances) and then try it again.

same result, even with fast startup disabled. also, before the update, it was fine with fast startup.

I have a dual boot repair utility (from boyans.net) with a "repair bcd" function plus an "automatic repair" option, but i'm afraid that if i use it, it may render one of the installs unbootable.

It is pretty much impossible to make an install un-bootable (as long as you have a USB key you can boot from to fix it).

I don't know boyans.net but you could try it. It doesn't sound like a bcd issue though if you have the menu and you can eventually boot though. More like one or the other is locking something.

What versions are you trying to dual-boot? Is it EFI or BIOS?

SOLVED!!

The "powercfg -h off" command, run on both installs, worked. What this does, I do not know, but it fixed it. Thanks so much!

It turns off hibernation (and also fast startup). Having either active is troublesome in dualboot as after suspending first instance, booting the second and then the first again, it may not restart from where it thought it was. This is especially true if you mount any partitions from both.

Well, Initially after using the "powercfg -h off" command, and disabling fast boot in the bios, it worked. But a couple of days later, the problem returned. ???

SOLVED:

I realized that the 2 versions of Win10 on this machine were were at *different stages of being updated*

Fully updating each build solved the boot issue.

OS #1 is Win10 LTSB, a stripped down version of Win10 I use for high performance music applications. OS #2 is stock Win10 Enterprise for normal use.

The reason that running the "powercfg -h off" command only worked temporarily, is because I ran windows update on one of the installs after doing that.

I can now select either OS and boot into it without difficulty. Thanks for the help.