Is it possible that your USB cable to the computer is not good anymore? Have you tried all the USB ports? Have you tried running "Total Commander" from the SD card?
EDIT: Meant to say, have you tried doing a hard reset with the device? Also found this:
You might need Mio Transfer Hack. I think this program is an essential asset for altering / adding / deleting and copying files on your device.
Install Mio Transfer on your PC. It is on your DVD. Then copy mio transfer hack.exe into the install directory. Now you are all set. Turn your device on before connecting to your pc through the usb cable.