On the wiki page,
Additional suggestion for USB media install lines, the OpenBSD line is just how it works, but the MacOSX line is an optional (yet arguably more reliable) method.
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OpenBSD:
dd if=OPNsense-XXX-memstick-amd64.img of=/dev/rsd6c bs=16k
The device must be the ENTIRE device (in BSD language, the 'c' partition), and a raw I/O device (the 'r' in front of the device "sd6"), not a block mode device.
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MacOSX
sudo dd if=OPNsense-XXX-memstick-amd64.img of=/dev/rdiskX bs=64k
where r = raw device, and where X = the disk device number of your CF card (check Disk Utility) (ignore the warning about trailing garbage - it's because of the digital signature)
Thanks for the additions, I've copied your lines into the wiki page.
Cheers,
Ad