I had Win 8 and Ubuntu 12.04 peacefully coexisting on a Toshiba Portege Z935. EFI mode, Grub2, secure boot off, had to use "recommended repair" on Boot-Repair tool to get the boot working after original Ubuntu install. All happy for about 10 months until….
I foolishly accepted MS's suggestion to update Windows to 8.1. After the upgrade, each boot brought me to a "grub rescue>" prompt. Was able to do a lengthy workaround to get back to grub menu on each boot, but wanted it fixed.
So, I started messing around with Boot-Repair, repeatedly, until the box wouldn't boot at all (just get blacks screen with "Insert disk with command files" or some such).
Tried reinstalling Win 8, upgrade to 8.1, then reinstalling Ubuntu. Same result – wouldn't boot at all. Tried reinstalling 8 without upgrade to 8.1, same result. Tried installing JUST Ubuntu. Same result.
Finally, set box from UEFI to CSM, installed Ubuntu again. Now it boots! So, I'm guessing the UEFI is somehow hosed in the "firmware" by one or more of the actions above.
How to fix? I'm out of ideas, and I want EFI back. THANKS!!!