Why is Time Machine unable to back up to LaCie?

are you using any of LaCie's preloaded software on the drive (or installed such software yourself) that supposedly makes the drive "Mac ready"? if yes, that could be the cause of your issue. when you use software from just about any manufacturer for their storage device you run the risk of that same software not being compatible with macOS updates / upgrades. it can cause the software to stop working. at best this will cause you a headache. at worst it can render your data inaccessible. it is always best to use disk utility to erase and format a drive before use to remove any preloaded software.

if using such software, please see Erase and reformat a storage device in Disk Utility on Mac - Apple Support

Detourer wrote:

Thanks for the response and a suggestion to fix. I have to say that I got this message after running Disk Utility/First Aid on LaCie. Could that be part of the problem?

sorry, I'm not entirely sure what you are asking. but you should first erase the drive. then eject it. then when you reconnect the drive for the first time, it should ask if you want to use the drive for TM.

Is it formatted as APFS?

What error do you get?