I know there are plenty of similar questions but no solution worked. I installed Ubuntu on a new HDD and have Windows 10 installed on another SSD. Win 10 is installed in legacy mode and Ubuntu too.
For certain reasons there are 2 additional HDDs each with Windows 10 installed (they aren't bootable anymore - don't ask why please :D). GRUB does find these two Windows 10 installations but not my actual Windows 10 installation.
Neither sudo os-prober nor sudo update grub show my SSD Windows 10. The SSD is mounted and I tried adding it manually like proposed here: GRUB does not detect Windows
Strangely enough BIOS only shows me my Windows 10 SSD as boot option but I can manually boot the Ubuntu HDD.
Sorry if I didn't use the correct terminology.
EDIT: Fast boot is disabled.
