I started to remove a card from the D7100.  The card wasn't even out of the slot.  This message turned up on the little screen.



Hmmm
Looked it up in the manual and three books.  Then online. No luck.
Clear the cards and formatted in the camera.  Same message from img #1.
The big screen showed this message.  One I've seen before.  Blew out the slot.  Formatted the cards in the camera.  Same old shit.



and the little LCD kept blinking.
Keep in mind all these cards work fine in my lowly D3100.
Went online to get more info.  Got instructions to reset the camera.  One set involves two buttons on the outside and there is a reset in the menu.
Tried them both several times. Nada.
Now I'm getting irritated.  Going to the last resort.  Two of my new lenses need a camera with a bigger buffer anyway.
Decided to give it one last try before the Clarkson Method.



Pulled the battery and put it back in,  Put in the D3100's slow card in slot two.  My fast SanDisk card in slot one.  Viola! now all is well.

Anyone know what happened?

I don't know what happens but I'm curious now in case I ever run upon the issue

I took out the card in slot two.  Got the same message.
Wouldn't work with the card in slot one and slot two empty.
Roughed up the contacts on the slot two card.  Works fine.  For now.
Slot two card came from the D3100.  I don't remember when I got it.
I'll get a couple new cards Monday to see if that fixes the problem.

If not the D7100 goes to APS on Monday instead of the D3100.

Cards do go bad on occasion, so try a couple of different ones.  

I had a Lexar freeze up on me once, and had trouble recovering the pictures.  I finally got them, and then threw the card away.  And currently, I have a SanDisk card that seems to be running very slow.  Popped a different  card in the camera and all is well now. That SanDisk is going in the trash, too.

If they are questionable, I'll toss them out.  I'm not going to risk losing the pictures for the cost of a new card.

The card err was a problem with Canon too. It usually means there is a read error and has nothing to do with the card.

The cards work in the D3100.  No issue using the laptop to retrieve the images.  That makes me lean towards a camera issue.
No matter.  I have a bunch of running around to do Monday.
I'll get a couple new cards just for drill.  The old cards I'll use in the D3100.
It if it acts up while the D3100 is at APS, the D7100 can go east when the D3100 gets back.

I'm keeping my eye on D7200 prices in case this turns into an ongoing problem.

Enlighten me.  If I have to get new cards I might as well get the cards you're referring to.

That's the exact card I use in slot #1  
I'll get a couple in 64gb

Its still pretty easy to jam up the 7100 taking raw pictures, I've been able to force the issue but took over 2K pictures in Wyoming this summer without ever having an issue with it buffering.  

That is a big perk for the 7200, the 7100 was praised because it matched the same rate as the 7000 with much higher resolution pictures

Its even more fun if you don't have to go through the pictures, but my macbook air struggles with the raw shots anyway, and my desktop has a 4K monitor on it so its best if I go into my office editing fun

Memory cards can go bad just like hard drives do.
If you forgot to turn off the camera before ejecting the card, maybe that played into it.

Put the bad card into a card reader; do a low-level format on it from your computer to see if any badness is found. If the computer thinks it is good, put the card back in the camera and do the standard quick-format in the camera.

Personally, I would not bother with it. I would just get whatever you need from the card, and trash it. SD cards are pretty inexpensive today. At some point it will go bad again, and then you may not be able to read it.

That's the plan.  Got a couple new cards coming.  The D3100 isn't quite as fussy about cards(or anything else for that matter) so I'll use the old card in it.

Send an email to the manufacturer. Lexar and SanDisk have lifetime warranties on their cards for free replacements after jumping through some hoops to show the card has failed.

True, but to me probably not worth the trouble. in terms of time & effort, the cost will be about the same if you go and just buy a new card outright. If you have some of those high-speed professional cards it will probably be worth the trouble to get factory replacements since they can cost like hundreds of $$. For me personally, I just use the consumer cards which is more than adequate for my purposes.

I don't think it's the card.  It's the slot.

The card in #2 slot works fine in the D3100.  I tried several cards in the D3100.  All worked.
If I pull the card in slot #1, the D7100 thinks slot #2 is empty.
I tried slot #2 with several cards.  Same result.  Slot two seems to be dead.

For now I can do with one slot.

Keeping track of refurb D7200 prices if this issue can't be solved.  
Monsoon season is here.  It'll get shipped to APS after I play with my new glass.