m240 files getting corrupted?!

Hello I have m240 player with firmware 4.1.08a. it plays well but for some time past i’ve noticed that some files are dissapeared from players library,but when i connect it to pc i see those files,i can copy them to pc, but i cant play them or open in mp3 editing program! it says unknown format! can player modify files ?or it is the problem with defective flash chip? I reformat device but after some time the problem occured once again!Should I replace the device or it’s normal with sansa m240?