Aha, found one of the ghastly insect things in the DT 1.9 prefs panel, tab = backup. The option to backup to a different destination is greyed out and can’t be selected.
It’s a v minor issue, and only one at all because I’d like to be able to copy the whole DT folder in Apple “Backup” and not have to copy everything except DT’s own rolling backups (does that make sense?). But I’m used to having to remove stuff from Backup’s directory list with 1.8, so it’s no big deal. Take a long & well-earned holiday first. PS - 1.9, love it.