Get another SD card.
format it.... (fat 32)
put it blanc inside your TT 300.
Connect it to the computer and use the TT-Home program to get the latest software for your device.
Now you can test if you can turn it on and off like it should.
Remember at this point there are no maps installed.
If this fails you know its trash.
If it works (with err. no maps found) you know its oke.
then copy the map from the old SD card to your new one. and your done.
this way you keep the original (possible faulty) SD card intact.
Warning: SD card to buy/borrow should NOT exceed 2GB !! since higher capacity (SDHC) wont work in your rather old device.
Arnold, February 2009