That probably means something had the files locked so they couldn’t be deleted.
Could be Opus itself (e.g. reading metadata to populate columns or thumbnails) or a third party shell extension, or something outside of Opus like search indexing or antivirus scanning the file contents.
Thanks. Files were not locked by e.g. another application. The above command now works fine again on the external drive. BUT.. I meanwhile figured out that maybe(..) it was because too many tabs were open on that drive: switching from one folder to the other another tab was opened. At around 5-6 tabs I got the error. Closing a few tabs to, down to let's say one or two: no problem. I'll try to keep that in mind. Thanks again.