LRCAT is almost 2TB need tp reduce it

It maybe a good idea to consider rebuilding your Preview files. The volume involved will take quite some depending on the choices you make, could be a couple of days or several hours.

The link below leads to a useful article that can assist.

https://www.lightroomqueen.com/lightroom-performance-previews-caches/

Enjoy.

A few additional questions:

1. How many images are in your catalog?

2. How much space is the "Lightroom Active Catalogue 2020-v10 Previews.lrdata" taking up?

3. In the Lightroom Classic>Edit>Catalog Settings>File Handling tab, what option have you got selected for "Automatically Discard 1:1 Previews"?

All the indications are that the previews folder is in excess of 1.5TB, which is huge. So just trying to find the cause.

Looking at your screen-clips, you must have a very large Catalog of images, resulting in a very large Previews folder (size not shown). It will be the Previews that are filling your external drive. As stated your Catalog file (by itself) is ~10Gb.

For Example - My Catalog is 1.163Gb  (~1/10th the size of yours) and my Previews are 114Gb for ~61,000 images.

With a very large Catalog of images you may be needing a larger Drive for the Castalog (with the Previews).

One small point:  Windows systems have (since time eternal) reserved A: and B: drive designations solely for the old 5" and 3" Floppy Drives from the 80's.

It might be advisable to rename the external Drive to something like- P:Drive  (P for Photography).

Having it as an A:Drive may be contributing to erroneous 'Free space' reports.

After a Drive rename, you will need to relink the Catalog to the new Drive name by a [Double-Click] on the .LRCAT file in its location on the P:Drive.

Here is the Drive Rename method-

https://helpx.adobe.com/lightroom-classic/kb/lightroom-loses-photo-location-after-drive-letter-change---windo.html?red=a

From the Screen capture you have posted it looks like your Catalog file is about 10 GB. I would suggest you check the Backup folder and see if are collecting a multitude of Backups. Each time you do a Catalog backup a new file is created. You can keep a few of the most recent backups and discard the older ones. Just decide what you are comfortable with. It's best to save to an external drive so you do not lose the data if your main drive crashes.

In addition the Previews file a significantly larger that the Catalog file, particularly if you are also creating full-size previews.

Just so you are aware the Lightroom Classic Catalog file is not very large and it does not contain image files or copies of image files. The Catalog file has an extension .lrcat, (dot LRCAT) please show us a screen capture of the folder that contains your catalog.

See the screen capture of my Lightroom sub folder which is located in the "Pictures" folder on my Mac by default. I have marked the three Catalog files that I have on my system, you will see the size of the catalog files are relatively small.

The Folder structure that actually contains all my image files is presently 794 GB.