You might want to try putting it on the primary IDE channel. LG's instructions say to put it on secondary because they assume you have an IDE HD connected on primary. They don't take into account the use of SATA hard drives. Also, you can try jumpering the drive to CS (cable select). There are a few computers where DVD writers will only work detect correctly as UDMA2 if they are jumpered as CS. I don't know why.
Lastly, you don't have any other device sharing the IDE cable with the DVD writer, do you?