Gigabyte Brix: how to get past the BIOS?

[briareos_kerensky]

Sounds a very stupid question, isn't it?
I've got a Gigabyte Brix, a mini-ITX barebone with everything including but RAM and HDD.
I turned it on for the first time, set the BIOS as I wanted (essentially changing data and time), stuck a USB drive with a Windows image in it, rebooted and...BIOS again.
I've tried four different Windows images, two Linux distros, a rescue image, but still no boot from the USB drive (all bootable, and confirmed working on other PCs). The drive itself isn't recognised all the times, so I've installed the OS on the Brix's HDD using another computer, reinstalled the HDD in the Brix and...BIOS again.
The HDD is always recognised by the BIOS, but if I hit F12 to enter the boot menu, it's not shown on the list. With the USB drive inserted (and with the drive recognised), hitting F12 and selecting the USB drive as boot once again brings me to the BIOS.
Gigabyte documentation, FAQs, and KBs are more barebone than the Brix itself: does anyone have direct hands-on experience with these barebones or any suggestions on how to have it boot from any device?

[briareos_kerensky]

Scratch that, I've been able to boot from USB (I had to disable the secure boot options from the BIOS), now onto understanding why the HDD doesn't show up in any OS I tr to install and why the Brix is so stubborn on not booting from it whatever is on.
Argh.