I have a number of external USB hard drives, none of which are bootable.
But, the rEFit menu shows all of them, in one huge long list, extending off the edge of the screen.
The two viable options (Mac Hard Disk and Bootcamp) are not even visible, since they are pushed off the left.
Plus, rEFit takes a long time to boot up because it scans all of them.
It would be nice if there were an option to limit the menu to only show the internal drive partitions, not any USB drives; perhaps even in some settings file that we edit manually. This may be doable now, if so I just don't know how.
Check the refit.conf file in the /efi/refit folder. You can disable booting from external drives.