The "Move" option moves them (i.e. they are no longer in the original location, just the new location). The "Copy" (or "Copy as DNG") makes a copy in the new location but leaves them in the original location. In this case the copy in the new location are the ones that LR considers the correct ones (i.e. are shown in the folders panel). If the destination were the "Pictures" folder (either by choice or default), and you used the "copy" mode with a destination of "Pictures" then in LR the images would show up as being in the "Pictures" folder on the C: drive. However, if you looked in Finder you would see the images in both Pictures on the "C" drive as well as in their original location on the external drive. This is exactly what you describe as the situation.
The Folders panel only shows folders (and if enabled, parents of those folders) that contain images known to the currently open LR catalog. When you used the "copy" mode of the import dialog, it instructed LR to COPY the images from the external drive to the Pictures folder and to use the copy in the Pictures folder as the ones to be imported and to ignore the copy on the external drive. So, once the import was done, LR was unaware of the copy on the external drive and thus does not show the folder in the folders panel as there are no images in that folder that LR knows about (it knows about the copy in Pictures, not the copy on the external drive)
A collection is sort of a "virtual" folder but does no change the physical location of any images. Even though you have them in a collection, this does not alter the fact that the underlying images are still physically in the "Pictures" folder. If all your other images are on the external drive, I think it's somewhat risky to have a few of them in the Pictures folder. For example they may not be backed up along with those on the External Drive. Let us know if you need advice on how to get LR to know about the ones on the external drive instead of those in the Pictures folder. There are several methods one can use and the choice depends on how much (if any) work you've done on these images since they were imported.
Dan