Dual boot Ubuntu and Windows 8: UEFI vs Legacy mode

Solution 1:

Setup your USB boot media the standard way, and it will be able to boot in either UEFI or legacy mode. It's possible to convert your legacy Ubuntu install to UEFI, (add the /EFI/ubuntu bootloaders, change a few files like fstab, but it's far easier to just reinstall in UEFI mode when you have the proper media. How you can boot the media depends upon the way the media has been set up -- you set it up just for legacy boot, so it can only boot in legacy mode. Be careful when you reinstall Ubuntu -- DO NOT select a "reinstall" grub option, that will wipe the whole disk.

Whatever you do, back-up your important files on the Windows side first. Bad things can happen with a simple typo. Oldfred's boot-repair suggestion should be an easy way to convert the Ubuntu install to UEFI. It is just an extra step to switch modes to access the other OS, I don't know of any harm it could cause.