I can boot from my USB flash Kingston 2G in DOS and QNX.
This is my kernel:
sys/boot
$ boot -v
sys/Proc32
$ Proc32 -p 1024 -l 1
sys/Slib32
$ Slib32
/bin/io-usb
$ io-usb
/bin/Fsys
$ Fsys -a -r 16384
/bin/Fsys.umass
$ Fsys.umass fsys -n hdu
/bin/mount
$ mount -p /dev/hdu0 /dev/hdu0t77 /
/bin/sinit
$ sinit TERM=qnx
you should have 77 patrition on your flash with QNX.
you should also run
/boot/sys/Slib16 &
in sysinit file. Slib16 started in “.boot” before, but your “.boot” file will be too big with Slib16 and will not work.
I also think it will work on USB floppy if you mount appropriate device, but I didn’t try.