How to get back Windows on the grub menu?

[Bo Berglund]

I installed Ubuntu Desktop 20.04.5 on a Lenovo IdeaCentre 3 PC a few days ago as
a dual boot to Windows 10 (se my other threads about tha effort).

To begin with the resulting grub menu contained  both Windows and Ubuntu.
Then I used GParted Live (booted from the USB via Windows) to copy in an Ubuntu
Server 18.04 partition from my original server.
When I restarted the computer to the Ubuntu desktop it did some grub update too.
After this the grub menu contained both versions of Ubuntu in addition to
Windows 10.
But it also contained the items from my backup disk used to transport the
partition between the two PC:s.
So I unplugged the backup drive and then from the Ubuntu server I issued:
sudo update-grub

Now the extraneous items disappeared from the grub menu and I had Ubuntu
desktop, Ubuntu server and Windows.

But for some reason when I reboot today the Windows entry has disappeared and I
really need that for f/w updates to the hardware and to be able to boot from
USB.

So:
How can I get back the Windows 10 entry on the grub menu?
The efi partition is still intact on the disk when I inspect it using GParted in
Ubuntu desktop.
I have tried this inside both Ubuntu versions:
sudo os-prober
/dev/nvme0n1p5:Ubuntu 20.04.3 LTS (20.04):Ubuntu:linux

So it only finds one os only...
But there are 2 on the disk and on the grub menu.
But no Windows...


-- 
Bo Berglund
Developer in Sweden