I stupidly decided to update the BIOS on my homebrew dual boot (Win 10 / Ubuntu 17.04) system, spec:
ASUS Z170-P D3 m/b 8GB 1x 250GB SDD containing Windows and Ubuntu partitions 1x HDD data only drive, NTFS Booting in legacy BIOS mode (I think).
All was ok before the BIOS update (which was a problematic process - I experienced a known looping issue with Internet BIOS updates) but it completed successfully first time.
Once I resolved the BIOS boot loop, the system booted straight into Windows, no GRUB stage. I checked the HDD boot order, the Windows Boot Manager partition was 1st on the list, so I changed it to the SDD drive that contains GRUB. Now, it boots to GRUB but I've the option to select Windows has disappeared, the only option is Ubuntu, which boots correctly when selected.
I presume the BIOS update has somehow reset the BIOS settings, but I don't know what. Fast Boot is disabled, as it was before. Secure Boot was enabled after the BIOS update, where I don't think wasn't before, but I've changed to disabled and I still don't see a Windows option at GRUB. I've tried grub-update, to no effect and I'm now at a bit of a loss as to how to restore Windows to GRUB.
I've tried changing a few settings in BIOS but nothing seems to bring Windows back, though I admit my understanding is flaky. If I change BIOS CSM to UEFI mode, Windows boots directly (no GRUB / Ubuntu). I guess I installed Ubuntu in BIOS mode but I've no idea how to either change things so it boots in UEFI mode, or change the BIOS settings so that GRUB sees Windows again.
Any suggestions from the crowd?