In the case of FireWire, if you don't have a FireWire source device connected, you see a window open just as you launch Final Cut Pro, telling you that you have a "missing FireWire device." Troubleshoot your connections with your camera or deck, and keep clicking Try Again until Final Cut Pro opens. Sometimes it's just a matter of turning on a device or unplugging and then plugging back in the FireWire or other connection, such as an RS-422 cable, to secure contact with your Mac.
You see this warning if you go out of FCP and back after turning off the device and leaving the Log and Capture window open.
I heartily recommend that you consult Apple's qualified device list at http://www.apple.com/finalcutpro/qualification.html before you invest in a device that won't work with the current version of Final Cut Pro. Don't be a guinea pig. Note too that some devices might need an extra or different capture preset. If they do, and if they have been qualified by Apple to work, this website shows you what you might need to do to get your particular device to work with Final Cut Pro.
Most RS-422-controlled devices work with the proper capture card or D/A converter and therefore might not be listed in the device list. Ask in the online discussion groups about any proposed addition to your system in this regard. Again, don't be a guinea pig. You'll find that most devices in this caliber of machine have been used successfully with Final Cut Pro.
If you are using an RS-422 device, be sure to select the appropriate RS-422 protocol in the Capture Preset editor. The RS-422 protocol varies somewhat between Sony and Panasonic devices. Simply experimenting with the various serial control settings in the Device Control settings will probably reveal the proper settings you need to use with your particular machine.