I went through 3 that said they could work with the "y" cable and using the usb charging port on the power brick...non of them worked like I needed...thy would read but not write, write but not read or just get stuck in an endless loop of starting and stopping but doing nothing. I have had no issue with the powered external dvd reader/writer though. works flawlessly.
i have an external burner that i actually had to dust off the other day and burn a CD for someone. it came with the Y cable but worked just fine on my SP4 with just using a single cable from USB port and burned and played.
I have an LG, Model SP80NB60 DVD Writer I picked up from Best Buy for a totally unrelated application that works just fine with it. Doesn't require a Y cable at all, the stock cable is a single USB connection. $30