M500 Storage card issue

[Guest brightlingsea]

My storage card (512 Mb Kingston) reports it has 85.88mb free. Yet when I try to copy a music file to it, or even just move a file already on the card to another folder on the card) Active sync/Windows says

"Cannot move {{FILENAME}} There is not enough disk space"

there is 88mb and the files are both well under 4, so what gives?

I can get music onto the card with Windows media player synching, but that is far from ideal. AND it should work just dragging and dropping - shouldn't it?

Thanks