"Card Full" error when the card is in fact empty...

Make sure that the SD slot is not dirty, check inside with a light and you may want to blow it out.

Download the H1 driver for windows & try different usb cable........

I want to practice through key board by singing along with with the help of SD card. How can record my favorite songs as karokee in SD cart to play in the instrument.

A very important thing when using these recording devices is to PROPERLY close the media. I would not panic at this point. Search for a forum regarding the Boss Micro BR and ask there as somebody may be able to help you recover the data. Read up on your manual about shutdown. I have Tascam recorders and I prefer to stop the recording and then when it is done I turn power off before removing the media. You may have to use a special procedure when introducing the nearly full card... even forcing it to load.

Re-format the card. Use it in a PC if necessary.

Does it say ERR NO FILE? If so, then read on. The reason you have this error is because you don't have a folder on your sd card named MUSICDAT. Solution:

1. Insert your sd card into computer
2. Make folder called MUSICDAT.
3. Then, transfer all files inside the folder (make sure your songs are in MIDI format).
4. Safely remove your sd card.
5. You're ready to put it in you Casio Keyboard!

If you still get the error, take time to read you manual. Read the "using the sd card" part. The folder name might be different for your Casio Keyboard (because mine's a CTK-5000).
...and IF you still get the error, perhaps the songs you meant in your question are MP3 songs (like the ones you play on the iPod)? Then, that's a different topic... :)
Well then, good luck!

Make sure the batteries are good... some people try to use re-chargeable batteries and because of their lower voltage they cause problems. First thing is to try new batteries.

Format the new card in the unit. Often the delivered format in a new card is not compatible.

The sound file has used all available space on the memory card. Either put in a different card or purge the sound file(s) on the card so you can record new files.

It is either that you put an incompatible SD card in it. (Believe it or not, many devices will not work with certain SD cards), or the card reader has failed. Try a Scandisk SD card or other popular brand and see if there is any difference in the messages. Otherwise I would say the card reader is faulty.