Problem with hd-nw5, erasing songs when connected to sonicstage

Hey, not sure if anyone has had this problem. I can transfer files fine onto the hd5 but if i want to add some more songs or whatever it erases all the stuff i previously put on there, i.e it doesn't come up on the sonicstage directory or on the hd5.

Also, do you have to use sonicstage or can you just use windows drag and drop system like i did with the iriver?

Thanks.