OK, I
think I have the cause & cure.
Cause:
Canon says the software [firmware?] bundled with the Canon Rebel XT "is not fully compatible with the latest version of OSX." I can continue to automatically bring up iPhoto or use ImageCapture, in any case. Thanks, but I want LightRoom 3. Canon recommended that I set the controls in ImageCapture to specify opening LR3 when a camera is detected. I did, but it was doing this before, so I saw no difference. Also, I'm not sure it was doing it reliably, before or after this change.
This software incompatibility might explain why the Canon Rebel XT was not loading into LightRoom 3, but would not explain why it was not recognized by the Finder to add to the Devices list. The Canon wouldn't get added to the Devices list in OSX 10.4.11, either.
ImageCapture could easily see the images on the camera, and could copy them to a HD without difficulty. But it's not LR3, and nothing convinced the Finder to add the camera to the Devices.
Cure:
What I did find, eventually, is this:
I set ImageCapture to open itself when it recognizes a camera.
I jack in the camera, and ImageCapture opens.
THEN I open LightRoom 3. It comes up with the present images in the library.
I invoke on the Import command by button or keyboard command. It opens the import display,
with the images from the camera.
I set the LR3 import settings as I want them. I make a habit not to erase the camera, until all is said and done, and disconnected from the computer.
I click the Import button, and it does!
SUCCESS!
If I have it open ImageCapture first, that seems to persuade LR3 that images are on the camera.
Sometimes iPhoto accomplishes the same end.
I went through this sequence a number of times, starting from neither software open. I tried different sequences - LR3 then ImageCapture, camera first or last, etc. The above sequence worked reliably; the others' responses confused me.
Those of you with similar Canon uploading problems, see if this helps any. If you don't have ImageCapture, try it with iPhoto instead. Just open it first.
Does this work around work?