You can probably just use uboot to boot riscos. You'll need to look for your boot partition, I'd suggest doing a 'mount|grep /\ ' (to search for the device mounted as root), then 'lsblk /dev/<deviceid>' where deviceid is the value returned by the mount command less the last one or two characters (try both, that command won't change anything, it just lists blocks', to see how many partitions are on that media, and if it's mounted, where. Then if it's not mounted, mount is as /mnt ('sudo mount /dev/<deviceid> /mnt'), then observe it it contains a boot.txt or any autoboot.txt. If it contains one of those, that's where you need to set up the riscos boot parameters. Unfortunately I don't know enough about risc os to suggest what to put. but it's worth finding out what you can see on your pyra following these instructions.