Need help erasing Disc's

I’m not sure if this is a problem with Nero or my cd burner (qsi cd/dvd sbw242b).When I try to erase a rw disc Nero says the disc is not rewritable.I’ve tried Nero 6 & 7 and have the same problem in both and I’ve tried different rw disc’s.I can burn cd’s without any problems but can not erase them.