I am confused by your terminology of "collection folder". You need to be only considering "Folder" locations. (Collections- to use the Lightroom terminology- are NOT physical folders, and cannot be 'moved'.)
The folder for uploads from a mobile device is set in the preferences dialog, and you can change this at any time.
So first, move the FOLDER of iphone photos from the current location to the external drive, by dragging the folder in the Folder panel of Lightroom. Then update the Location in the Preferences/Lightroom Mobile tab by choosing the new location.
I hope this answers your question.
Thank you, wobertc. That does help explain the setting I need to change and why I should not be able to move the collection. However, I don't see the FOLDER of iPhone photos in the Folder panel of Lightroom. Under Folders in Lr, I see the Macintosh HD, but do not see a folder that contains my iPhone photos.
I see two hard drives in the Folders list - one shows my iMac's hard drive and one is my external hard drive. Under the iMac, I only see two folders - one is 2015 and one is a named folder that only has 5 photos in it while the collection
I do wonder why that photo that holds the collection isn't showing as a physical folder in Lr. I can only see the collection.
I do see the preference you referred to, so that's great for changing that location going forward.
Any thoughts about why I don't see the physical folder in Folders?
Additional Information: It appears that at one time I did move the location of this folder that had my iPhone photos to my external drive. I see a folder of the same name in the Lr Folders panel on the left under my external drive with a subset of iPhone photos. However, it is not appearing under the Lr Folders panel under my iMac internal drive.
So I wonder if I can just change the preferences to point to that physical location and physically move the files using Finder. Will Lightroom folders pick them up? Or do I need to "reimport" them after I move them using Finder.
And I wonder if I should physically move the files first in Finder and then change the Lr preferences.
Thoughts on that?
A Collection, Any Collection whether or not is it synced with LR Mobile, is just a List of images you want to be Grouped together. There are no Folders on your drive that stores what images you have placed into a collection.
As for the Sync folder for images synced back to your desktop computer from your phone follow wobertc advice for moving it.
If I'm understand correctly, you want to first move the images to the new location, keeping the folder levels/hierarchy intact. Then in Lightroom find the top level of the missing folders. It should have a question mark. Right click on it. Then "find missing folder". Navigate to the correct folder, click ok. Assuming the hierarchies are intact, all subfolders should fall into place also. Here's a tutorial on same: Move folders around in Lightroom | Adobe Photoshop Lightroom CC tutorials Don't reimport, Lightroom can do this easily!
Very helpful video, joefry99. That combined with wobertc seems to give me all I need to get things straightened out. Thanks!