Yes, adding files to a partially-filled disk. The DV-420v does a really great job of reading DV-RW disks burned that way, but can't see past the first "session" on DV-Rs. Also, it works just fine with any DV-R I've burned as a data disk. No problems there at all. I'll just have to change my habits . The Panasonic being replaced by the 420 had just the opposite behavior. It read multi-session DV-Rs perfectly, but really choked on DV-RW multisessions, sometimes with constant stops and freezes to the point I'd have to turn the player off and back on to recover. It didn't manage layer switching on DVD-9 double-layer disks very well either, often just freezing at the switching point.