I’ve finally successfully installed ROCK on a Gigabyte BRIX. The difficulty was that I already had Windows Server 2012 and Debian installations on a partitioned mSATA 128 SSD. Gigabyte motherboards appear to have a continuing inability to see a USB boot drive if they can see any fixed boot drive. I had to format the mSata drive using an external mSATA-USB casing and then play Gigabyte USB hokey pokey a few times before the USB ROCK installer showed up as a boot drive in the BIOS.
After that it was plain sailing. A 2TB internal SSD for music was formatted and available in the ROCK/Data share without a problem.