Can you let us see the UEFI, CSM or Legacy BIOS menu choices now in BIOS, attaching camera snaps in your reply?
Ask Toshiba tech support how to get these menus to offer choices. Does it need a BIOS password set up first?
I'd also ask them how to solve the problem with Win8 which they are required to solve or repair/replace the laptop.
What you need to avoid is telling them you've done anything to modify it. You can ask them about Dual Booting to see if they are willing to help you but never acknowledge that you have done so or are doing so, or made any changes, as they may be scripted to deny Tech Support if so.