Originally Posted by Gertjan
Possibly the SD card got corrupted. You can check and fix the card in a PC.
When the problem persists, and the same occurs on different SD cards, you may have hit an issue where the system refuses to mount the SD card after erroneously files are created when no SD card was inserted. You can work around this by (re)installing a firmware update.
Take the SD card from the device, copy a firmware update to the card using a PC, insert the card back into the device and hit the reset switch. After the upgrade process, the SD card should be mounted correctly.
Thank you very much for your kind answer! I'll try it just tomorrow because now it is midnight here and I don't have SD card reader now. And I'll post the result at this thread. Thanks again!