Enabling BIOS/CSM/legacy support was a mistake. You now have Windows installed in EFI mode and Linux installed in BIOS mode. Most computers make it difficult to switch from one boot mode to the other, so such mixed-mode installations are difficult to manage.
To fix it, install an EFI boot loader for Linux. As detailed on the page to which I've just linked, several are available; but as part of the normal process of EFI boot loader installation requires that you be booted in EFI mode, you can't do this from your current Ubuntu installation -- at least, not without jumping through some extra hoops. There are several ways around this, one of which is:
- Download the USB flash drive or CD-R version of my rEFInd boot manager.
- Prepare a USB flash drive or CD-R from the downloaded image.
- Boot the rEFInd medium you've just prepared. This may require using your firmware's built-in boot manager or going into the setup utility and disabling BIOS/CSM/legacy support. If you use the boot manager and see two entries for your boot medium, select the one with "UEFI" in the description string.
- In rEFInd, you should see an entry for Windows and one or more entries to boot Linux kernels. Select one of the latter and Ubuntu should start. Note, however, that if you used a separate
/boot partition, you'll need to hit F2 or Insert twice, rather than Enter, to launch Ubuntu. When you do this, a simple line editor will open, in which you must enter a
root= specification pointing your kernel to your root filesystem.
- In Linux, install the boot loader of your choice. Two relatively easy ways to do this are:
- Install the rEFInd Debian package or PPA, as described on the rEFInd page. This will set up rEFInd as your default boot program.
- Run Boot Repair again. This should install the EFI version of GRUB. Note that this path leaves more to chance than the preceding one; at this point you really don't know that the way Boot Repair sets things up will work, whereas you know that rEFInd will boot Ubuntu.