OH yes! i was waiting for this, i have seen ,tons of liteons with the same problem.Seens to be a particular issue with the 7xxx series.
Somehow, laser get stuck off the right position, so the drive doesnīt read any media.
What i did to solve :
Worm gear and motor - Worked for some drives around 70%
Lubrification - No success
Another laser - No success
Another PBC - No success
Flash to stock - No sucess
Put in other Xbox 360 to test - No success
Replaced the white arm - No sucess.
The error seens to be random,replace the Worm gear and motor works almost everytime, but some drives are stubborn. I noticed some DVDs need to be read from the edge of the disc, when this happen, sometimes the laser get off the right position.
So what i did, i measured where the laser get stuck, so i put a or "bumper fix", or whatever u wanna call it, itīs really simple to do it, but need to be measured exactly to not damage the unity.
How this works : The bumper act as a "limiter", so the laser canīt get of the righ position, it workīs 100%, my first fix was made almost 4 months ago and still working fine.
Itīs a simple fix and donīt damage anything at all. i came up with this after i almost gave up to find a solution,nothing seens to work. Can it be a software bug ? Missing calibration data? who knows...
I have used the rail from a old cd-rom drive. Check the pic.

I hope this fix can help you guys with this problem.
Ps : People around here have a more simple solution called "tap that ass" - this fix consists in slap the xbox in the back, works miraculously well lol.
This post has been edited by rafinhalive: Oct 28 2010, 01:25 PM