(See images.) I'm trying to temporarily remove this plank of the deck to access the space beneath. The plank is being held in place by a type of fastener I'm not familiar with. Do you know what kind of fastener this is and how I should remove it? Additional Details
I will need to put the plank back in it's current place after I'm done, so I can't damage the plank too much. I am willing to destroy the fastener, if necessary, and replace it with a new fastener of the same type or a different type. I don't yet know what is under the plank; it could be concrete, wood, brick, or something else. I tried cutting a slot in one of these fasteners and then I turned it with a slotted screwdriver. It just turns in place and does not come up, so I think it's either not a screw or it's completely stripped. Should I just grind the head off with a Dremel or is there a better way? Thanks in advance.