Hello, I purchased a new MacBook Pro about 3 weeks ago, the 15" 2.53GHz with a 250GB HD and 4GB RAM. The stock HD is a Fujitsu. I want to upgrade to a 500GB HD, so I purchased a WD Scorpio Blue since it essentially had rave reviews. I put the drive in, used the OS X install CD to reinstall Snow Leopard, and then transferred my settings by putting my stock HD into a USB enclosure and using Migration Assistant. The computer did not act normally; it would slow down every few minutes and I would get the spinning beach ball, even when performing just ordinary tasks. Also, it was louder and the fan seemed to be working harder. I returned it and bought a Hitachi 7pk500, the 7200rpm 500GB Travelstar. At first, this drive was even worse than the WD. It installed successfully, but there were then filesystem errors that I had to use DiskWarrior to correct. However, the drive was still not performing well; after a reboot, the computer hung on a blue screen. When booting and holding option, I noticed that the drive was called "EFI Boot" and not "Macintosh HD". Lastly, I reformatted the drive and used Carbon Copy Cloner to clone the drive. This seemed to work (instead of EFI Boot, the drive was called Macintosh HD), but as soon as I started doing a Time Machine backup, I noticed the machine slowed down and essentially paused (beach ball and the backup would stop) every minute or so for about 20-30 seconds. Finally, I put the stock HD back in, and I'm not experiencing any issues at all. Sorry for the rant, I just wanted to make sure I provided as many details as possible. Does anyone have any idea about what may be going wrong? I know that people are able to successfully use both WD and Hitachi drives without problems. Thanks!