Yes Johnny Flash, you're right. I'm mistaken by the illusional effect of the Keynote which the new iMac introduced by Phil Schiller and many posts afterwards. All was saying that the options are 3 TB disk, 1 and 3 TB Fusion drive etc ... I always thought that 3 TB disk is an option for both iMacs. After your answer I controlled and found that 3 TB drive is not an option for the 21.5" iMac, but for the 27". It's so simple. Thanks.
It is not easy to find an answer. Because you or somebody should sacrifice his adapter:) It's a recent thing and not easily break. But I have an idea. Clip a cable-tie horizontally to your adapter. Cut the tie end a little longer enough to drill and hang to your key-ring. a 5 mm tie width with a 2 mm hole would be good for the purpose. I cannot add a photo but cable-tie holds the device firm enough. Maybe a drop of proper type of glue will secure more. If you don't like it, nothing is harmed than a cable-tie!