Windows 10 CU destroyed my second S.O.

Hello,
I am a little crazy now.

I just updated from Anniversary to Creators via Windows Update.

Everything seem to be OK.

In my system I have TWO Windows 10.

The second is HIDE from the first (simply hidden letter).

I tried to launch the SECOND Windows 10 (still Anniversary) and... cannot boot.

So I went in the disk manager and... there is the partition, there is the label but... ZERO BYTE!!!

Please, tell me what I can do!!!

I cannot believe that CU destroyed the dual boot of my computer!!!

Update:

I just discovered that the data is still there. Luck!

But, what is the easiest way to repair the dual boot? It is possible from Windows itself?

I find some command lines to do but they are related to MBR but I have a GPT disk.

UPDATE:
I ran Macrium USB.
I did "FIX BOOT".
It found nothing.

Than I ran DISKPART and I assign letters the each disk with an OS.

Reboot.

Then Macrium find all the Win10 installation.

I did FINISH and it REDO the BCD.

So I try to launch the SECONDARY Win10 (anniversary) and it WORKED!!!

But after a REBOOT it was down again with the same error.

The FIRST Win10 (CU) works Always.

I don't know what to do.

UPDATE:
If in the bios I select BOOT MENU (F9)
- Launch EFI FILE
- HD (only one is selectable)
- BOOT
- BOOT64.EFI (not the correct name)

I can run my SECOND OS!!!

But If I reboot again that error.