I replaced my 200gb 5400 hard drive in my 15" Macbook Pro, Early 2008 model, with a WD Scorpio Blue 500gb 5400 hard drive. I cloned the drive with Carbon Copy Cloner and everything seemed to go smoothly. But after I began using the Macbook in earnest, I noticed a delay in response at certain times. If I type in a word processing program, I will type five to six words, the cursor won't move, then I'll get the rainbow spinning wheel. After five to six seconds, the line will appear quickly, letter by letter, until it catches up. If I keep typing, everything is fine, but once I stop for 20 seconds or more, the same thing happens. Being more observant, I began to notice hiccups in other programs as well. I'll give a trackpad input and the spinning wheel appears, then the program catches up. I then began to listen closely to the hard drive. After 20 seconds of activity, it sounds like it slows down, even going to sleep--although I'm not sure if that's truly what it's doing. It's only when the hard drive seems to slow/stop that this lag in input happens. If I keep giving inputs before the slowdown, everything is fine. Here is what I've done to troubleshoot. Perform a verify disk from disk utility. No issues detected. I've unchecked "put hard disk(s) to sleep when possible." Still have same problem. I've booted from the old 200gb HD which I encased in a disk enclosure and the lag time doesn't occur when using it. Any ideas? Is there something else I should have done with the BIOS or something else when installing? Also, I bought the HD from Amazon. If the HD is defective, does anyone have experience to know whether WD will send me a new HD or do I have to go back through Amazon to get a new one. Thanks for the help.