If I'm booting from USB, is it possible to edit SYSLINUX menus to select a distribution on an internal hd? I don't mean deferring to the internal boot partition, but selecting an internal partition distribution from a USB bootloader. GRUB?
It wouldn't be mounted yet, so I couldn't assume a path to the internal drive.
Does the booted os have to be on the boot partition?
If this is possible in your situation and if you're doing this in SYSLINUX, then you can always change the root= value in your syslinux.cfg file. Most often, you would link it to your first SATA hardrive, which I believe makes the value root=/dev/sda0 or root=/dev/sda1.
Example:
DEFAULT linux
LABEL linux
SAY Now booting the kernel from SYSLINUX...
KERNEL vmlinuz.img
APPEND ro root=/dev/sda1 initrd=initrd.img
