I bought a xbox 360 elite, it still have the microsoft seal so it has never been opened but the drive is not reading disks, I was wondering if I could just buy a new drive and replace it with no problems playing online with it?
You'll need to figure out what kind of drive it has: What you can do is use a hair dryer on the seal to peel it off. The old silver seals are a little bit trickier but can be done. The new white paper seals are really easy. Once you get the seal off you can put it on some wax paper. If the drive does in fact need replaced, you have a couple options. You can dump the firmware and flash it to a new drive. Or buy the same type of drive and swap PCBs. If it is a BenQ or LiteOn it will require desoldering and soldering 6 wires. This info has nothing to do with the DVD drive but I thought I would throw it out there. I would check out the dashboard version your 360 has. If it has a dash of 2.0.7371.0 or lower, you have a JTAG'able console. If it has been update to the 2.0.8xxx.0 dash, you aren't able to JTAG.
what if I just replace it with the same model without flashing it? are they able to detect it? are they serial/PN bound?
you can't just "swap" drives... each drive is "married" to the console. Your existing (faulty) drive is meant to be installed in YOUR console. Its got a firmware key that matches the key on your console. if you swap/switch drives, the keys won't match, and your xbox will only allow you to play DVD's, NO GAMES. Just like leerage said, you will either have to swap PCB's (the motherboard inside the DVD drive - which is where the key is contained) or flash the new drive with the same firmware key as your old drive. Flashing is very easy, i'm a complete noob, but after reading on here how to do it, and watching some videos, i've been able to flash a few drives now.
if the pcb is fried, then the drive is dead. I don't think they "fry" that easily though.. if your drive is not reading disks, then you should read up on that problem, and try to fix it. It could be something as easy as cleaning the laser, or something a bit more complicated like needing a pot calibration. Explain whats exactly wrong with your system, with more details, what messages are you getting? And if the seal is still intact, check with microsoft to see if its still in warranty.