It's driving me up the wall, but I can't find a solution to this. I've got a Raspberry Pi Zero, and I'd like to put Risc OS on it. Grabbing the Risc OS 99meg install file and using Win32DiskImager to SD card it onto the Zero did nothing. Black screen. Installing NOOBS did work however. NOOBS has a Shift key boot option that should allow selection of different OSes - but it doesn't show any alternative OSes (apart from Raspbian). I think this is because the PiZero has no wired connection, and the wifi dongle hasn't yet been booted up. So there's no way to choose an OS.
Very grateful if anybody has a solution - or perhaps the Raspberry Pi Zero simply can't run RiscOS