Currently, dropping storage units works in a fairly bizarre way. If your inventory is full, both it and any overflow from your now-empty inventory are thrown on the ground normally. If your inventory can, after removal of the storage unit fit that same unit, it moves it into inventory as expected. However, if your inventory is not full but would overflow if the unit were removed, you can't drop it - there's a message saying that "Inventory capacity will be insufficient."
I'd suggest making it so that dropping a unit does it whatever the state of your inventory - if possible, with a confirmation repress. If you don't want that, never letting you do it if capacity would be insufficient would work as well, but right now with it depending on just how overfull the inventory would be seems like a problem.
To reproduce: Open a new game and kill the scrapyard scavenger until it gives you a storage unit. Equip that, and try unequipping it with 3, 4, 5, and 6 items in inventory. It should succeed, fail, fail, succeed, respectively.