Install on USB

That's bug 1173457. The installer always puts the Ubuntu bootloaders on the first disk, usually sda. But the good news is that the bootloaders are just files, you can make a 300M partition on the USB, put a FAT filesystem on it, and copy the files from the hard disk's EFI. One other bug, the USB is a "removable media", so the default bootloader used is actually in /EFI/Boot/bootx64.efi (assuming 64 bit architecture). To fix that, just copy grubx64.efi and shimx64.efi from /EFI/ubuntu to /EFI/Boot and then rename /EFI/Boot/shimx64.efi to /EFI/Boot/bootx64.efi. Save the old bootx64.efi if you want, it's just a copy of the Windows bootloader. Those fixes will allow the USB to boot on a UEFI system. Since it has nothing in the MBR, it will not boot on a legacy system.Setting it up to boot both ways is possible, takes another 2M partition labeled GRUB_BIOS (?), and another grub-pc package install, but leave that out as perhaps not needed.