Thanks. Yes I already do that for all backups.

Support? What Support? I posted this problem and another on HDM 15 12 days ago and zero response. Anyway I've given up with HDM 16.5 and reverted to 15 so a waste of three licences.

I did try to see if 15 would restore a 16.5 backup but it said it wasn't supported. Don't recollect any mention of that though or that the backup format has changed in 16.5. 16.5 can restore 15 okay.