Some consumer DV cameras have a recording mode called SDL, which increases recording time on mini-DV cassettes by reducing the track width and slowing down the tape speed. In some cases, trying to capture from a camera in SDL mode in iMovie and Final Cut Pro can result in the application not recognizing the camera or not capturing any video.
Symptom
In iMovie, the camera is not recognized. In Final Cut Pro the camera is recognized but no video can be seen or captured.
Solution
Using the camera's menu system, switch it from SDL mode to "SD" or "Standard" recording mode. After this you may need to turn the camera off and on before Final Cut Pro or iMovie will recognize it.
Additional information
SDL is an open industry standard specified by ISO/IEC 61883 and 61834. Other long play formats are not affected by this issue.