working theory: the bios’s legacy usb support, and specifically system management mode. coil runs in pure real mode and its EDD reads work, the bios reads the stick happily at boot. but helix runs in protected mode, and calling the bios means dropping back through a thunk, and that round-trip is where it dies. on a machine this age legacy usb lives in SMM: the bios traps disk interrupts and does the transfer in a mode invisible to the os, assuming a clean real-mode cpu, and our PM excursion breaks that assumption. the tell: teletype (int 10h) works from the thunk, disk (int 13h) hangs. teletype doesn’t touch usb, disk does. photo is the bios setup screen, where i went hunting for a legacy usb toggle which i did not find ;3c