Just to close this post & also hopefully help others in a similar situation:
The MP3 player is part of a huge scam.
The player is only infact 2Gb. It has been flashed & had a new set of instructions installed to make it think that it is 4Gb in size, which does show up as a 4Gb removable drive when plugged into a PC.
This fully explains why some music files became corrupt, when the storage went over 2Gb.
The people who do this to the MP3 players know that a 4Gb MP3 player will sell for more profit than a 2Gb player.
There are 1000's of these imitation nano MP3 players on the market, mostly on aution sites & they all claim to be double the capacity that they actually are. I have read many forum posts about this scam & most people seem to have ended up with a 512Mb or 1Gb player, so the one i have been playing with is one of the lucky one's, as 2Gb is still quite a large storage device.
The moral of the story:
Buy the real thing, as you know for sure what you are getting!