Problems in opening copied files to USB Disk

Dear Sir,

I have a generic USB Flash MP4 player of 1GB memory. Copying files (including music files) from PC to player doesn't give any problem. But, when opening some files show data error / currupted message.

Action taken: a) ran chkdsk, b) defragmented disk, c) Formatted

Outcome: Nil, but after many trial and errors methods I discovered that files copied for first 120 MB is fine and have no problems but, files copied there after doesn't open giving data error/currupted message.

Can anyone help in understanding the problem and give the remedy to solve.

This shall be a great help.