Hello Sam,
If I try to boot from Open Firmware, I try "boot usb1/disk@1:,\\grub.img" and the error after quite a long pause is "can't OPEN: usb1/disk@1:,\\grub.img Can't open device or file" Of course I can't rule out the possibility that I'm using Open Firmware wrong, after some searches of this list and on the Internet I didn't specifically find a description of the boot line and I'm really not an OF wizard.
GRUB is located at /boot/grub/powerpc.elf most of the time. The command you're looking for is `boot usb1/disk@1:,\boot\grub\powerpc.elf`. You can also see what's on it with the command `dir usb1/disk@1:,\`, adding directories at the end to go deeper.
My G4 was unable to even recognize USBs in Open Firmware, but the fact that yours showed up is a good sign. That means it should be able to boot from the drive provided it can read the filesystem.
I use variations of these commands on my G3 all the time with success, and some others have managed to get them to work on their G4s. The NetBSD installation guide for macppc[1] was a wonderful resource about all these Open Firmware commands (I believe it was even cited in the Debian Wiki as being a massive help).
[1] https://cdn.netbsd.org/pub/NetBSD/NetBSD-9.2/macppc/INSTALL.html Regards, -- Ben Westover
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