I have the YAMAHA DVC6770 (aka C750) and have had the same problem. I sent the unit in to the repair/exchange center, as I figured I had a defective unit. I don't think the locking is related to the disk, because I would put the same disk back in and not have any problems. In addition, I had the unit freeze in the middle of a Dvd, when unplugging unit and starting over at same spot there was no problem. I considered returning the unit (I had mine for around three weeks also), but I really like the sound quality. Maybe they had a bad production run recently? The turn around on the repair/exchange is supposed to be 5 business days, mine has been out for a week, so I should be receiving unit soon.