If you're going to move the photos using your operating system to the new 3TB HDD, then yes you would have to open every catalog you have and reconnect the photos inside of LR (or use LR to move the photos, which I do not recommend, again you'd have to open each of your five catalogs). By the way, this is a fantastic argument for having one catalog.
A simpler approach is to leave the photos on the existing 1TB HDD, and put new photos on the new 3TB HDD. No moving of existing photos required, so no chance of things getting fouled up.
To answer your other questions:
3. The backup dialog has nothing to do with "catalog file will then be updated"
4. Yes, the catalogs remain in place, unmoved. The backups should NEVER go on the same disk as the working catalogs.
5. Moving (or copy and paste) the catalog files (.lrcat) and previews (.lrdata) does not automatically make new linkages to photos, one has nothing to do with the other
6. Backups should never be on the same disk as the working catalog
7. I don't know — but if your not having any speed issues in Lightroom, don't worry about it
8. Shouldn't be affected
I think you need to do some detailed reading about how catalogs work and about how they reference files. The basic idea is that your Lightroom catalog must know at all times where the photos are located and what the files are named. If you perform moves and renames inside of Lightroom, then Lightroom still knows this information. If you perform moves and renames in your operating system, then you must specifically tell Lightroom the new information, as explaiend here: Adobe Lightroom - Find moved or missing files and folders, Moving catalog files does not affect the linkage between Lightroom and the photos, the catalog files can go anywhere you want them to, but the best place to put them is on your fastest drive, if you have the space and don't need portability. One catalog is highly recommended.
DJ has you pointed in the right direction.
However, if you are going to move your old pictures to the new drive, do that move using Window's File Explorer, not Lightroom. Lightroom is not a file browser and it does poorly at moving large numbers of files.
After you do the move using File Explorer, you will need update each catalog by going in to the Folders panel in Lightroom and reconnect the moved folders by "find missing" or "update folder location" on the top-most folder(s) that was moved. Lightroom will reconnect everything at that level and what is below it.
(Why are you using multiple catalogs?)
Thanks. I'm clear now on most of the points.
(1) In respect of moving files within Lightroom vs within explorer and then re-establishing the link - the google searches I made all recommended to do within Lightroom. However both replies so far indicate that I should do this in explorer and then re-establish the links. I'm comfortable with doing that - but is this the recommendation based mainly on the fact that I have a large number of files (and folders) to move (and LR is "not a file browser"?
(5a) With my planned set-up (OS/software on SSD with lots of space; and separate HDDs for storage) is the best location for my catalogs on the SSD?
(5b) Now the part I'm most apprehensive on. If I want to move my catalogs (I assume these are the *.lrcat and *.lrdata files) from the 1st HDD to the SSD then what is the best way to do this? As far as I can work out the only way to do this is by moving the files in explorer. But you say that "the catalog files (.lrcat) and previews (.lrdata) does not automatically make new linkages to photos". So next time I open up LR I assume the catalog will open but it will not be able to find the photos. So will I then need again to help LR reconnect with the folders/files in the new location? I put again in bold because there are two different operations going on - firstly the movement of original files from 1st HDD to 2nd HDD and then the movement of the catlog files from the 1st HDD to the SSD. So I just want to check that I'm correct in needing to do the reconnection twice.
Why multiple catalogs? I started with LR2 about 7 years ago, and 32-bit system with very limited RAM and a large number of files. I found LR to be incredibly slow and googling at the time recommended having far fewer files per catalog ... so I split my photos between major life periods like young free and single, marriage and birth of children, and work. And having done that I've stuck with those catalogs without thinking about it. Now my current catalog has 17,000 photos at an average 6MB each. With newer more up-to-date hardware specs I have no noticeable speed problems with this latest catalog of 17,000 photos. My other catalogs probably total around 17,000 photos too (with an average of about 4MB each).
With my current catalog at 17,000 photos I had been wondering whether it was about time to start a "new" current catalog. With 64-bit OS and LR running on an SSD and significantly more RAM it seems you are saying that these days there is really no need to have multiple catalogs. Is that correct? At the moment I only have around 34,000 photos occupying say 200GB and I'd guess this increases by 3,000 photos per year.
If the recommendation then is to merge some/all of my existing catalogs into my current catalog: would I be best to do this merge before or after the various file/catalog movements proposed in my original post? Or does it not matter?
Got you. Coming from the real early versions of LR, yes, multiple catalogs was a recommendation for performance way back then.
These days, the modern database systems work wonderfully where size is not a problem.
You could merge the catalogs back together.
If you have only the three, you could just stay with the three and let them grow as they will.
You might merge the "single" and "married" and leave the "work" separate.
I use multiple catalogs for my sports and events shooting, mostly for archiving purposes. However, all my personal photography is in one catalog.
For moving a small number of files, or a single directory, doing the move inside LR works very well.
For moving large number of files, or trees of directories, LR starts to fall down at times. Doing the actual file move using the OS tools (Finder or Explorer), which are specifically designed for that purpose, is much faster and less error prone.
The catalogs store your file references as absolute file paths, not relative paths. Thus moving the catalog does not break any of the file links. If you want to move a catalog to a new directory or a new drive, you just use Finder or Explorer to move the "*.lrcat" file and its matching "*preview.lrdata" folder. If you have LR configured to open the last used catalog, it will complain that it cannot find it, since you did move it. Just go to the menu /file/open/ and navigate to the new location. Just make sure the new location has READ and WRITE access.
As to putting the catalog on the SSD, I have not seen any conclusive results that stated real performance gains. It is possible that there are conclusive reports (one direction or the other) and that I have just not seen them.
Thanks again ... all very helpful. I'm now confident I can go ahead without screwing things up.