Transmission getting stuck when adding single torrent

I am having problems connecting to Transmission after added a single torrent.

The torrent starts and first when the torrent is done - I am again able to connect to transmission.

I can see in the “Summary Information” that the Pi is not being “overloaded”, so I don’t understand why the Transmission is acting like a “big process”?

I am writing to an external 8tb harddrive.

It’s a Raspberry 3 - newest model.