simono , .7z is a compressed file. You'll need 7-zip to uncompress it, the installation file is here
http://www.mympxplayer.org/7-zip-ver...6-26-df12.html
If the formatting goes well and there is no hacked memory, then i can only think that there might be a problem in the USB cable.
If your player is 2GB and below, FAT should work well, FAT32 would be recommended for larger capacities. However, using FAT32 for a 2GB should have no problems too.