When I turn my camera on, it says 'no SD card', even though there is an empty SD in it. I don't know if it has to do with me deleting EVERYTHING the last time I loaded all the photos off.

The camera is saving images to the internal memory - change to save to card

May I suggest the battery was very low. The Nikon delete routine shows a bright display after the file is deleted, this may take a bit of power. May I suggest you work on your files on your computer? That is, that the SD card out and work on the files on the computer. Now I suggest charging the camera and taking out the SD Card and then turning it on. If it works try it again with the SD card in.

The problem most likely is caused because you are deleting EVERYTHING. Next time, instead of deleting everything, format the card in the camera instead.

Hello bro..
Do you check your SD card? It is work ?
My solution:
1. Off Lumix camera.Until LCD was off. Press center of card to remove it.
2. Use card reader to connect your SD card to pc/netbook/laptop..
3. Check full capacity for SD card, it is empty(zero) or have it their mount.
4. If empty (full,used,free is zero) ,try reinsert card again and don' t touch the terminal.
5. If still empty, that mean, your SD card was corrupted.
6. If not empty, backup all data in SD card.
7. Format the SD card using FAT32.
8. Checking Disk.
9.Insert back to camera, and format SD card using camera menu.
10. Take picture. and see if save on SD card or not. comment back here. if succes or not.
11. Don't use any battery except original from panasonic. It will affected to product.
12. Don't remove battery or card before lcd was off light.
---------------------------------------------------------------------

Look at your memory card. SD cards have a slide switch along one edge. The position farthest from the contacts locks the card, protecting it from writes. The position nearest the contacts unlocks the card.

To resolve this problem:-Turn off the camera and insert a new memory card.
-Transfer or delete pictures from the memory card.
-Change the storage location to use the internal memory.

I've been studying the problem for several minutes and actually downloaded the manual from the Nikon site to confirm a suspicion. With the camera software denying the transfer yet other "Windows" application are makes me believe that there is or are corrupt files on the memory card.
The Nikon D90 is a "Semi-Pro" type camera and feature packed, however if you have been deleting files and not formatting then the memory card has become saturated with empty files (deleted) what needs to be done each time you down load is format the memory card. Formatting cleans off all the old empty files, restores the memory card to a fresh medium and allows the camera and cart to communicate between each other efficiently. Also one other thing that might have happened is that the protect lock has been engaged. The camera SD card has a lock protection switch on the left upper side (with the letter facing up) this little switch need to be pushed to the top edge towards the gold contacts. I'll supply a diagram, there is also another thing is this a new memory card? if so did you format the card to the camera before you used it? Also your camera has some SD card memory capacity restrictions these are shown on page 242 of your manual.

Diagram

the camera is meant to be used with a SD memory card - the internal memory is very small and probably will only hold 5-10 pictures. empty it just by deleting the pictures. memory cards are cheap and available everywhere. good luck!

Use a cardreader to download your pictures to another folder you create on your desktop. Then, format the card in your camera to erase everything for a fresh start.

Take the sd card out and hook the camera to your computer to delete the internal memory or do it on the camera display then turn the camera off and insert the sd card and turn the camera back on.