@bizcocho85 you might be doing the upgrade mode wrong,
don’t worry it tends to happen with beginners,
I have written a trick of how to enter upgrade mode precisely,
it works everytime and I never skip a beat,
yup, its power by the computer, and that isn’t an issue at all,
while flashing, the chip barely draws any power, so it is never an issue, but if possible connect to a USB3.0 port for ample power delivery
that isn’t possible, I presume you are asking like a GRUB loader ?
with mainline uboot, I think you can cancel booting from the eMMC and switch over to external SD/USB, but I am not completely sure, maybe @hyphop would know something about it…