Another method of doing this so the parameters are permanently set is to edit the syslinux.cfg file in the root directory of your flash drive once you've created the UNetBootin install.
Just open this file in a reasonable notepad app (I use notepad++ under Windows and gedit or vim under Ubuntu), look for the entry or entries you wish to modify and add in the following text before the "--" part of the "append" parameter for that entry.
Code:
cdrom-detect/try-usb=true
Once done, save, eject and then boot your box with the now fixed installer
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