hard drive replacement not mounting

The major difference between the 2 drives is the one iFixit sells is a model hs12yha that turns at 4200 RPM drive and the one on Amazon is a model hs12yha/a (blow up the picture for the actual model on the drive), which is revision A of the hs12yha, that turns at 3600 RPM drive. There will be a performance difference any time the laptop needs to access virtual memory. So you will experience slower boot up times and more drumming of the fingers between clicking on a program to open and when you cam use them with the 3600 RPM drive. I really don't see a compatibility issue since at least 1 review on Amazon speaks of having successfully installed it in a MacBook Air.

Which one to buy depends on how you use the computer, how much you use the computer and how much you value your time. If you are a power user, with multiple applications open all the time, or use memory intense programs on a regular basis I would suggest the 4200 RPM hard drive. If you are a regular user, not using it for graphic design and don't watch many flash videos I would say the 3600 RPM hard drive is a very feasible way to go.

Please understand I am not advocating either direction. I am just trying to give you some food for thought to enable you to make an educated, economical, decision that will best suit your needs and wallet.