First you should try to perform a 'low level format':
Download and unpack the
firmware for your player (SP-version).
Disconnect the player from PC.
Press Enter for 20 seconds (just to make sure it is turned off)
Press and hold "Play" and "<<" until you see a purple screen "low level formatting... please wait..."
the screen will turn black and a green screen "no resource... please update resource again..." will come up.
Now you can connect your M6 to the PC and copy the 'resource.bin' to your player. It has to be the resource for the firmware that has previously been installed- if you are unsure take both 'm6.EBN' and 'resource.bin'.
If it doesn't work you'll have to use DFU tools.
You can follow mbarry's '
Guide: Fixing your "Bricked" Player' to do this.