I believe you want to work on a separate set of images on a different catalog and once done, you want to merge it to the Master Catalog.
You can create a new catalog and import the images which you want to edit separately.
Once done, please follow the below instructions to merge the catalog to master catalog.
1 > Open Master Catalog
2 > Click on File > Import from Another Catalog
3 > Navigate to the new catalog created earlier and then click on Choose.
4 > It will provide you a new dialog box with few more options for the images in the new catalog.
Please follow the below link to understand the options better.
Here is a video tutorial to merge multiple catalogs
Please let us know if this helps.
Please feel free to revert back for any further assistance.
Basically you are looking for the "export as catalog" and "import from catalog" features.
The catch is that you want to make sure that the directory structure of the images is maintained.
The way I would initially attempt this would be to:
copy the desired images off the external drive to the internal drive of your computer without changing the directory structure. (Copy whole directories.)
In LR using the catalog on the external drive, select the images you just copied, "export as catalog" without the "include negative files" option.
On your computer open LR, create a new catalog, "import from catalog" the catalog you just exported.
In the Folders panel, "find missing" on the top-level folder that you copied from the external drive.
Edit as you will.
Export your edits by selecting the previous group of images, "export as catalog" without "include negative files".
Open the catalog in the external drive, "import from catalog" for the catalog you just exported.
In the Folders panel, check for any "missing" folders due to the import.
This all works great in theory. You will want to test this using a small number of files just to iron out the wrinkles.
Thanks very much for your quick and helpful reply. I'll give this a go and, if I may, get back to you with any follow-ups.
Thanks ManicJoe, that's helpful. The one bit I don't follow is when I am exporting my edits back from the temporary catalog on my HD to the master catalog on the EHD.
You say "export. by selecting the previous group of images, 'export as catalog' without 'incuding negative files'".
If I understand you, the images at this stage are on the HD and the edits in the temporary cat, so don't I want to return to EHD *both* the temp cat (i.e. merge that with the master) *and* the pics? Hence shouldn't I select 'include negative files', when heading in that direction?
LR never changes the original files. It does non-destructive editing by storing the edit commands in the catalog and replaying those commands every time you want to view image (or export the image). Thus, the original files on the EHD and on your internal drive are exactly the same and do not need to be copied back.
The other catch with "include negative files" is that you may lose the original directory structure as LR copies the files around.