So here's a follow up on my own question for those of you who are thinking of going with an aftermarket 5400rpm 500GB hard drive for your unibody macbook pro.
Bottom line: each of the aftermarket drives has tradeoffs that you have to decide are worth it for your particular application. My main concern was not necessarily drive performance; instead, I was interested in getting a bigger 500gb drive that was just as quiet or quieter than the stock drive. I'm extremely sensitive to noise, so this was priority one for me. Given that there are manufacturing differences among all drive makers, my experience isn't necessarily generalizable for all of the makes of drives out there on the shelves. It's quite possible that I just happened to get slightly louder drives, though it's also not likely.
Note: As you've all noticed, the unibody macbook pro hard drive sits right beneath a very thin piece of aluminum on the right handrest. If you put your hand on top of this area in a completely silent room, you'll note that the whirring sound of the hard drive is dampened considerably. Because aluminum resonates more than plastic and the fact that our computers are very thin, hard disk noise is more apparent once ambient noise around you is gone. This is also the case for faster 7200rpm drives that may cause more vibration in the unibody. So my comparisons are for environments that are very quiet where hard drive noise is obvious. Anyways, I'll compare the replacement drives I tried below. The following is a key for my observations:
IDLE NOISE: Sound that the drive makes when not under any load and spinning at idle. This sound is the constant whirring (like an ocean) that the drive platters make.
HEAD PARKING: This is the somewhat aggravating ticking sound that drives make (usually more often under OSX) when the heads are parking to save energy. This sound is like a random (usually 5-10 second) apart ping pong ball type, single "tick" sound. It only presents itself when the drive sits at idle. Some drives are louder than others with respect to this sound. Stock apple drives are just awful when it comes to this. An OSX application called HDAPM (
http://mckinlay.net.nz/hdapm/) can eliminate this sound entirely but with the downside of possibly decreasing battery life. Use at your own risk.
LOAD NOISE: This is the sound the drive makes when something is written to it or it is being accessed by the OS. This sound is usually a muted clicking noise that tends to be almost inaudible unless the task is heavy on disk access. You should be familiar with it.
SPEED: I wasn't interested in this, but I ran some tests using SpeedTools utilities. This is the same suite of test programs used by barefeats.com.
Comparison below:
1. Stock Hitachi 320GB 5400rpm Hard Drive:
IDLE NOISE: This drive, believe it or not, is still the quietest of the bunch.
In a completely silent environment, idle noise is whisper quiet. You can hear it from about 2 feet away. It's not entirely unpleasant. The whirring is constant with an almost imperceptible high pitch whine. I have pretty good hearing, and this drive's idle whirring noise is only audible when the room is completely silent (rare). It barely makes more noise than the virtually silent fans at 2000rpm.
HEAD PARKING: This is where stock apple drives just suck. The Hitachi drive makes a pretty loud "tick" sound when it parks its heads at idle. This sound bothered the hell out of me until I found the hdapm application. Some people are more bothered by this sound than others, but the Hitachi made this noise more frequently and more audibly than the other two drives in my comparison.
LOAD NOISE: This drive is also pretty quiet under load. You can hear some slight clicking when it's working, but it's nothing that would distract you.
SPEED: Quickbench clocks this drive at 51mbps average read/write. Pretty sucky
2. Seagate Momentus 500GB, 5400rpm Hard Drive:
IDLE NOISE: This drive is perceptibly louder than the Hitachi. You can hear the drive from about 3 feet away in a silent room. The whirring isn't too high pitched, so it blends into the environment as white noise very quickly. I did perceive a slight whine, but I acclimated to it quickly. If I had to choose a 500GB drive, I'd probably choose this one based on idle noise and speed benefits alone. Again, this 500GB drive is louder than stock though. So don't go thinking you'll get a quieter drive.
HEAD PARKING: Worlds better than the stock Hitachi drive. I actually could not hear any head parking "tick" sound using this drive. Seagate has done a great job making this drive virtually silent in this respect.
LOAD NOISE: None that I could hear. Quieter than stock.
SPEED: Clocks in at 76mbps average read/write. Nice.
3. Western Digital Scorpio Blue 500GB, 5400rpm Hard Drive:
IDLE NOISE: I wanted this drive to knock my socks off acoustically, but it just didn't. I didn't hear any whine from this drive's platters, but the idle whir was noticeably louder than the Seagate, and it tended to oscillate its volume randomly. This actually drew attention to the drive and made it hard to tune out. It sounds similar to ocean waves coming and going. The drive gets louder and softer over and over again. No good at all.
HEAD PARKING: Like the stock Hitachi, this drive had a very perceptible head parking noise. It did not present itself as often as the Hitachi's (thank God), but it was particularly noticeable when it happened regardless of ambient noise. I would probably use HDAPM on this drive as well.
LOAD NOISE: A little louder than the Seagate. I'd say on par with the barely perceptible noise the Hitachi makes when under load. It's fine according to my standards.
SPEED: Quickbench clocks the drive at 74mbps read/write. Still faster than the sucky stock drive.
There you have it. If you want a faster drive with more space, I'd go with the Seagate (assuming you don't want an even faster 7200rpm drive). Ultimately, I wanted to compare the 500gb drives to my stock 320gb. I don't need the space now, and I ended up sticking with the stock Hitachi drive because it's so very silent. Your requirements and hearing will likely be different. But if you prize as quiet a laptop as possible, you probably will sacrifice a little silence for a faster/larger aftermarket notebook drive. Again, manufacturing tolerances will vary for the macbook pros and aftermarket hard drives. So take my observations as one person's opinion among many possible. It is likely, however, that your computer may sound the same with these drives in it. And lastly, everyone has different hearing levels/sensitivities. So you might not notice or care about half the points I mentioned above. I hope this helps some of you who are thinking of going to 500GB but don't necessarily NEED the space right now