I had a Windows 10 Installation in Legacy BIOS Mode on my Primary Drive (125GB SSD) and a Secondary Drive (2TB HDD) as a Data Drive (Both Partitioned as MBR).
I created a new Partition on my secondary Drive to install Ubuntu on to.
I installed Ubuntu in UEFI Mode (My mainboard seems to support UEFI and Legacy Simultaneously, because I can choose between "UEFI" and "Legacy + UEFI", at least that's what I thought). I created a swap partition, a root partition and an EFI Partition where I told the installer to place the Boot Loader into.
After all of this was done Ubuntu was Booting Fine but there is no Sign of Windows 10 when entering the Grub Menu (By Pressing ESC on startup).
When I try booting directly from the SSD (By changing the boot order) it says something like
Cannot Boot from this Drive.
I Already ran Boot-Repair, but it didn't bring back Windows yet. This is the paste it produced.
I'm running out of ideas and I really don't want to lose my Windows install.
Any help is highly appreciated.
Best Answer
So it appears using Boot-Repair and writing Grub into the MBR of the Drive you can turn the UEFI Ubuntu installation into a legacy one.
When Booting up Ubuntu in Legacy Mode now 'os-prober' finds the Windows installation and running 'update-grub' it now adds a menu-entry for windows and I can now Dual boot into Windows and Ubuntu.
The Conclusion and the Lesson learned from this is: Don't Mix UEFI and Legacy Mode OS installs.
Although I don't know if I were to remove the Ubuntu Installation again if my regular Windows Boot Loader would still be working. I don't actually think so.