Advice regarding transferring photos between external HDs

Transfer photos from Camera to Seagate. Then *COPY AS DNG* to the LaCie à in Lightroom and then *ADD* the photos to the catalog in LR as well.

I think this was very bad advice. Import the photos once only. (And make regular and automated backups of your catalog and your photo files.)

After this you can disconnect the hard drive (yet) and edit your photos within Lightroom. This was even advised due to sensitivity to hackers.

I have never heard this before. And this is a level of paranoia that I don't have. If you have a good antivirus and firewall, you should be okay.

I do now experience that my lightroom works very slowly and often crashes for a while.
question: Is this because of the hard disk?

Slow at what? Please be specific, different types of slowness have different causes. It may be because of your hard disk, and it may not be becuase of your hard disk.

There are a few things to understand about what it is that you're actually doing with the workflow.

If you start with all your photos copied onto the Seagate drive, you have made one copy of your images. Then, in Lightroom Classic, if you "Copy as DNG" to the Lacie drive, Lightroom knows about your photos on the Lacie drive, and that's all it knows. Any images you edit within Lightroom will treat the new copy of your photos on the Lacie drive as the "originals." The copies left on your Seagate drive are a good backup.

If you try to "Add" the photos on the Seagate drive to your catalog, you'll now have two copies of your photos (assuming you told Lightroom to ignore suspected duplicates). In Lightroom, you'll see two sets of folders, one under your Lacie drive and another under your Seagate drive. It seems likely to me that this will lead to confusion and potentially you working with the wrong images. I don't see any benefit in the "Add" step.

As to being able to work with the hard drive disconnected: this is a feature that is enabled when you "Create Smart Previews" during the import process. If your Lightroom catalog is located on your local hard drive, and it includes "Smart Previews," you can unplug the hard drive containing the original photos, and you can continue to edit using a lower resolution proxy of the image.

Note that working with Smart Previews will not allow you to send the original image to Photoshop, nor will you have access to the full resolution image for printing or sharing, unless you plug the drive back in. This page has a lot of good information about Smart Previews, and why and how you would work with them:

How to use Smart Previews to View and Edit Photos in Lightroom Classic