windows 10 iso on usb refuses to boot (dell optiplex)

I am using a windows "standard" ISO file: Win10_1511_1_English_x64.iso and I have verified its md5 matches the expected. The iso is mountable in OS X as a "UDF" format.

I then copy it "straight" to my USB drive, like:

sudo dd if=Win10_1511_1_English_x64.iso of=/dev/rdisk1 bs=1m

then plug it into my dell optiplex 780 (a little old, I know), enable botting from USB device in the BIOS, and all I get is:

No boot device available - strike F1 to retry boot ...

What am I doing wrong?