Plug-in the external disk and launch the command
If that command finds the other OSes on your external disk it should be possible.
A different possibility is to launch the BIOS boot-menu at boot time (I don't know the key for Dell to do that, but it should be displayed on screen when you start the machine) and choose the external disk for booting, since there should already be a bootloader installed. Keep in mind that this will not work with Windows on the external disk. Also, if you have installed proprietary drivers on the external disk's Linux install that don't apply to your desktop system there may be problems with starting the graphical environment.