My best educated GUESS is because the card is already 'booted up' by the external power supply,
that some of that power is getting back into the motherboard, so its 'partially'
powered up. It has no idea how to start up from that state.
Im also hoping that no damage to MB or GPU has been done by this...

A way that "MAY" work is to splice the two power supplies together.
Be sure to read this WHOLE post before doing anything.
And I also claim no responsibility if this causes damage either ,
but I have stated any risks THAT I KNOW OF.
Of course, disconnect from the mains while fiddling with cables, just in case.
You could cause the PC to boot up at a bad time. Bad for the PC, and your wallet.


The way I would try MYSELF would be to electrically join the two power supplies together.
To do this I would securely join one or two BLACK cables from each PSU together,
if you do choose to join two blacks, join them separately, as is 2 + 2, not all into one.
If you join them all together it wont 'spread' the power out like two joints would.
And then I would join the two GREENs together, so both PSUs turn on AT THE SAME TIME.

Thats it for the power wiring, now to electrically join the two CASES together
so both are grounded to the same voltage level. If you can 'bolt' the second PSU inside
your PC case, that will do the job nicely. This is an electrical safety issue!!
It is not needed to make the system work, but dangerous TO YOU voltages could
accumulate (sometimes in seconds). THIS IS CRITICAL!!
After that you are done with the wiring. But read on anyway, its important!

Point of note: MOST psu's only fully turn on when there is enough drain on them.
Thats why you had to connect a HDD to the PSU to get it to power up its fan.
If its fan is spinning, then its is APPARENTLY fully powered up. I have never
(in 15 years)come across a situation different to this, but still... hmmm.
You video card may only draw enough power for this when it starts having to work hard,
so that means the second PSU will only turn on LATER. This is bad, potentially harmful.
So I would leave your old HDD connected to the external PSU at all times, just so
the PSU fires up completely at the same time as the main PSU.


Well, thats all I can think of, mostly.
Other that with your PSU power wires, make sure to tape them up so they cant
touch anything, or drop into any fans.
And with the CASE strap, use mains rated cable if you can, or multiple wires.


Best of luck dude!

[EDIT] I suspect you could find and buy a cable to do this
joining, but thats just more cost eh? Your choice...