Memory Card Slot error message

I pulled removed the card from slot 2 and left one in slot 1. I received an error message saying that card in slot 2 may be bad, but the slot is empty. When I place a card inside it works fine. Any suggestions why this might occur.

I would pop out both cards and observe inside the slots with a flashlight. Compare both slots to be sure nothing looks bent and there is no junk in there, like a little piece of plastic that divides the pins on the card. I have seen these break off. If the slots look clear of any trash and nothing looks bent, I would then somewhat aggressively plug a card in and out several times and see if it clears. If not, it might be time to call Nikon. I think there are switch contacts in there that sense the card is in place and also a set to check the card lock. A little 'exercise' might release stuck switch contacts.

Was the camera turned on (instead of off) when you pulled the card out?
Have you tried using different cards?
Have you tried using different settings for the second card to determine if the error occurs with each option; Overflow? Backup? JPEG (when card 1 is for RAW)?