Oh - good points! Single player, while in build-mode on a ship I was building. The storage unit was placed on the ship. I had the inventory of the storage unit and the creative mode inventory panels open. Trying to drag/drop straight from one inventory to the other did nothing. I pulled the grey hull down onto the hotbar to swap out the gravity units. I then dragged the grey hull to the storage unit inventory, and it turned into gravity units when I dropped it. I tried moving the grey hull into other hot bar positions, but always received gravity units instead.
Curiously, I had no problem moving grey standard armor when I tested it.