Blu-ray drives that people buy externally for replacement don't tend to work in the PS3. There's been a lot of threads here about this.
Think you might just have to get Sony to replace it.
Motherboard for the drive under a metal cover?
Got one lol?
Hey dude, when I replaced my 80GB blu-ray drive I got the replacement drive here:
When I ordered (about a month ago) they were offering a £10 refund if you send them the broken drive.
I had to detach the motherboard from the original drive, attach it to the new one and then replace the drive. I couldn't find a guide for the 80GB specifically but it seems to work fine now.
So it is certainly possible to repair, just make sure you've got the right drive and that you make sure you're aware of any risks before you start
40GB PS3, or Fat PS3 isn't accurate enough for this though, you'll need the part numbers for the BluRay drive to be sure.
Good luck and I'm glad I'm not you! Replacing the BluRay drive is pretty risky. Make sure you're taking all the necessary safety steps and don't hurt yourself!
There's a compatibility list here:
Although yeah you might still want to bust your PS3 open and check the drive itself. After all, you'll only be opening it up later anyway.
As far as risk goes, replacing the drive itself isn't particularly risky - changing the laser possibly, but for the drive itself you shouldn't need to deal with any components of the PS3 apart from the blu-ray drive, its connectors and its motherboard.
You know, ground yourself, don't touch components more than you have to, be careful with the ribbon cables etc.
It may seem obvious but the more things you tinker with, the more likely things are to go wrong! Its easy to get distracted by the shiny things inside.
One last thing to those who have new PS3's or are less experienced: DO NOT TRY THIS YOURSELF. If your PS3 is under warranty Sony will replace it for free! Also, if you don't know your SATA from your FAT32 then you should probably leave the lid of the console firmly shut and get a professional to do it.