Fixing hard disk clicking / aggressive head parking on Mac OS X

I recently bought a Western Digital Scorpio notebook hard drive to put into my 2007-vintage Macbook Pro. Everything seemed fine at first. However, as I used my laptop, I noticed that it would frequently make a quiet clicking noise. At first, I thought that I had gotten a bad disk. However, after doing a little research, it became clear that this is a common problem. This clicking is a "normal" operational noise - it is the sound of the heads parking.

People say that you should just get used to the noise. However, this blog post makes the argument that every one of these clicks is killing your hard disk. Some people claim that this is related to the sudden motion sensor that's built into most (if not all) Apple portables. However, this is a red herring. The disk still clicks even if it is sitting on a table. It is the hard disk's own built-in power management that is causing the head parking. The disk's SMART statistics record the number of head parking cycles. If you want to see this for yourself, you can use either this menu extra or this command line tool (MacPorts). You are looking for the Load Cycle Count value.

To explain the problem (as I understand it), modern hard disks have some responsibility to manage their power consumption. One manifestation of this is to spin down the platters and to park the read/write heads. The operating system can influence the time before the heads are parked by setting the "APM Level" of the drive to a value between 0x00 and 0xfe. Each drive manufacturer is free to interpret this value as they see fit. Mac OS X seems to set a default APM Level for all disks, and I think this value is 0x80. This is fine with Apple-shipped disks, but not necessarily for third party disks.

But wait! Perhaps you have bought the same kind of drive that Apple ships in their laptops. Are you safe? Not necessarily. Allegedly, Apple flashes their own firmware onto the the hard disks that they install at the factory. That's right, you're not running stock disk firmware. My suspicion is that this firmware changes the drive's interpretation of the default APM level. Recently, there was a firmware update from Apple that fixed this problem on disks that were shipped by Apple. Unfortunately, you can't use this utility to flash the new firmware onto a non-Apple drive.

Right, so the two solutions that I see are either:

1. Write our own firmware
2. Set a different APM Level value

Obviously, option 2 looks much more attractive. Bryce McKinlay wrote a utility called hdapm for doing just that. He even includes a launchd configuration to run hdapm as the system starts. One thing not mentioned in the readme is that you need to get the permissions of the launchd config file correct. The file needs to be owned by root (preferably root:wheel), and must not be group- or world-writeable. I also changed the config file a little; here is my version:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
 <key>Label</key>
 <string>hdapm</string>
 <key>ProgramArguments</key>
 <array>
  <string>/usr/local/bin/hdapm</string>
  <string>disk0</string>
  <string>max</string>
 </array>
 <key>RunAtLoad</key>
 <true/>
</dict>
</plist>

The biggest change is that I removed the "LaunchOnlyOnce" and "ServiceDescription" keys. I didn't see a reason to load it only once, and ServiceDescription seemed undocumented. This solution isn't perfect, however. First of all, hdapm uses a seemingly undocumented back door to adjust the APM setting. Ideally, we would actually spawn a daemon that continuously monitors and adjusts the drive's APM level. I'm not yet convinced that Mac OS X won't override my setting. Still, I have been running with this configuration for a couple of days, and things seem to be working well.

I have an open support issue with Western Digital to see if they have a fix for this issue. If there were some way that we could change the way the disk behaves under OSX, we could forego the additional software, which would be great. I've also heard of a utility called wdidle, which allegedly lets you write new idle settings to the hard drive. However, I was unable to find any official site for this software, so I'm not using it.

Finally, I would like to thank two people. First, Doug Aghassi's post really explained the symptoms that he was experiencing and put me on the right track for solving the problem. Thanks, Doug. Also, Bryce McKinlay was kind enough not only to write the hdapm utility, but also to answer the questions that I emailed to him. Thanks, Bryce.