[solved] Can't image tos 9.3 on repaired optiplex 3000

Edit:

I was trying to use my old merlin / 5070 installer. Optiplex 3000 has its own different installer apparently.

dell-thinos-licensing-guide.pdf (delltechnologies.com)

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I've just got 2x optiplex 3000s back from the local repairer. 

One was motherboard (which won't connect to the WMS - I'll start another post about that, but I am trying to reimage it as well to see if that fixes it. Same issue as the other one...)

The other was HDD so now I have to get 9.3 back on it.

I've manged to get it to boot off the USB by disabling secureboot, but now it gets as far as 

Loading file initrd64.pxe...

And then reboots.