Downloading from FTP with Filezilla; The HDD got full so I transferred as many files as possible to 3x 32Gb USB sticks - The sticks are showing that they now contain around 29.5Gb, but the free space on the HDD is only showing about 5Gb - I did try emptying the trash folder, but no result there.
It seemed to work OK till the HDD got full, since the problem started; I've been careful to make sure I'm paying attention when I click "move" and not "copy", and I double checked that the top bar says "moving".
This looks a lot like a bug in Lubuntu!
Thanks for any help.