Marking catalogued photos in folder with other photos on external drive.

The process of finding and moving the files from the folder is relatively simple, albeit involves multiple steps.

I've attached an annotated screenshot showing the steps, etc.

A - You say that the files you want to relocate are in the same folder as some that are already in the LrC catalog. Therefore, your first step is import the other files into the catalog. It may seem odd, but stick with me. Rather than use Import, you should use the Synchronize Folder (1) option found in the Folders panel's context menu.

B - After the import is complete you should see that 'Previous Import' (2) in the Catalog panel has a file count. These are the files that were not previously linked to the catalog.

C - The next step is to use 'Create Folder Inside "xxxx"...'  (3) option found in the Folders panel's context menu. This creates a subfolder within "xxxx"

Note that "xxxx" is the name of the folder containing the files that you had originally cataloged and want to keep in LrC.

D - You now have an empty subfolder within "xxxx".

E - Now select 'Previous Import' and use 'Select All' (Cmd+A) to select all of the recently imported files, then drag the selected files to the newly created subfolder within "xxxx".

F - Now you need to switch to the Finder. 

G - Using Finder, locate the newly created subfolder within "xxxx' and move it to the location that you'd prefer the files to be (e.g. a different drive).

H - Switch back to LrC and select the newly created subfolder within "xxxx". Don't worry whether the subfolder or the files within it have a '?' as this just indicates that the subfolder/files are no longer linked to LrC catalog.

I - Click on the '-'  (4) button at top of the Folders panel.

J - Choose 'Remove' when the dialog appears. I've included a screenshot below for reference.

The file system can not know what's imported to a Catalog or not. The Catalog can not know about image files that are not imported. But you can use the Catalog to selectively do something, to just those image files that are imported.

For example: inside the Catalog's Folders panel, rename the folder from "[Name]" to "[Name]-spares". Then in the same panel, Add New Folder alongside, calling that "[Name]". Now drag all the image thumbnails that the Catalog presents, from "[Name]-spares" into "[Name]". Outcome: your imported image files are in a folder called the same as before, non-imported image files are separated out. 

Yes, laborious. Personally I would regard this as needless micromanagement anyway. LrC is already doing its job of distinguishing what's Cataloged from what is not. The file system does not need to separately do that too.

If (for example, to close off a previous year) you did want to regain disk space by thinning out the files, to only those that are imported - across many folders within that year - this is possible to do as a bulk operation, with an Export as Catalog / Import from Catalog sequence.

But for a single folder at a time, likely not worth the trouble. Depend on the proportion of keepers to incidentals, I suppose. I like to cull obvious duds at the import-from-camera-card so those won't even have made it into the computer. Just trying to design-out any future operations that can be avoided.

No, they are not marked, as this is also against the non-destructing principle.

Just an idea, you could rename upon import, and then you could decide imported from not-imported by the name/numbering.

To give the best advice, we need to know your workflow.