On a Dell M6800 machine that has Windows 8 pre-installed, I upgraded to 8.1, then I loaded Ubuntu 14.04 then Fedora 20 the only way I could, that is in "Legacy" BIOS Mode. BTW the Ubuntu install was unable to complete but Fedora did, and left all working, 2 bootable OSsen, with Grub2 from Fedora doing the boots, but Windows 8.1 is now "invisible".

I have run Boot-Repair but info is not useful. It tells me to switch to UEFI which, on M6800, doesn't work with this (too long-winded to explain why here).

I need to have the Grub2 configs "see" the original Windows 8.1 partition, with BIOS switched to "Legacy".

BTW various posts have said make switch to UEFI to boot from, say, USB-stick or DVD; this is wrong, you can't - UEFI mode only allows boot from the Windows partition, which it says has a Windows 8.1 on it, which doesn't boot.

Basically, if you have actually succeeded in loading Ubuntu 14.04 or Fedora 20 on an M6800, which comes with pre-loaded Windows 8, you will know how I fix this.