Emergency disk not booting IF another external disk is connected

[wbrells]

I recently installed Hasleo Backup Suite on an older Toshiba notebook system and had no trouble making an initial system backup onto a USB-connected external HDD. I then made an Emergency USB recovery disk and it boot fine AS LONG AS THE USB BACKUP DISK IS NOT CONNECTED. If the backup disk IS connected, the emergency USB disk will not boot - it flashes for a few seconds and then the notebook screen just shows a flashing cursor at the top left of the screen. (There is no error message.) Clearly, this isn't a good situation since I can't do a a system restore if the backup disk isn't available! Any thoughts/suggestions would be greatly appreciated!  Wayne