I only have an SRX650 sitting here, I put junos-srxsme-12.1X46-D55.3-domestic.tgz on the 650, and did the snapshot to the USB drive, plugged into the 240, it tried to boot from internal. Went into U-Boot and tried usbboot and it says * No boot device *. The SRX650 won't boot from it either though. But it seems like it was built correctly:
root> show system snapshot media usb Information for snapshot on usb (/dev/da0s1a) (primary) Creation date: Aug 12 18:36:11 2016 JUNOS version on snapshot: junos : 12.1X46-D55.3-domestic Information for snapshot on usb (/dev/da0s2a) (backup) Creation date: Aug 12 18:38:10 2016 JUNOS version on snapshot: junos : 12.1X46-D55.3-domestic
I'll try another USB stick this afternoon.