Depending on how you use your Mac, you may start finding that the hard drive is being used quite a bit, even when you are not running any applications. This can sometimes happen even when systems have not been used extensively.
There are a number of reasons why the computer's hard drive gets accessed at high volume, with some instances being normal, expected use, and others being potential faults or the result of having installed third-party software.
Spotlight indexing is perhaps the most common reason why your Mac's hard drive is churning away. The system checks for when changes to the file system have been made, and invokes the notorious "mds" process to index files for Spotlight's use. Usually, Spotlight indexing does not take very long; however, this depends on the number of files on the drive, and how frequently they've been changed.
This will be done for all locally attached drives, and may take longer for Time Machine drives, because of their large number of hard links, among multiple updates to various files.
If there is an error in Spotlight's indexing, the easiest solution is to reset it by removing the spotlight index at the root of the drive. This can be done in two ways: By the Terminal (ensured to be more through) or in the Spotlight system preferences.
In using the terminal, you will need to open the Terminal application and then perform the following procedure:
Type this command, followed by a single space:
mdutil -E -i off
Drag your hard drive to the terminal window, so the full path to its mount point gets entered.
Press enter, and repeat this for all locally mounted hard drives.
Repeat the above procedure for all drives again, except change "off" to "on" in the command.
In using the Spotlight preferences, open them and go to the "Privacy" tab. Then drag all of your hard drives to the list and close it down. Wait about a minute to ensure the system recognizes the drives in the list and turns off indexing for those volumes, and then go back and remove them from the list.
After either of these steps are performed, you should see the pulsing dot in the Spotlight "magnifying glass" menu extra, indicating the drive is being indexed.
Beyond Spotlight, Time Machine is another process that can make high use of the hard drive. Time Machine's drive use is very sensitive to the number of files being regularly modified, and size of those files. As a result, if you have large cache and other temporary file locations for programs you use (i.e., a "scratch" folder that you use for clips when putting together movies, or a temporary folder to contain large file downloads that take hours to complete), Time Machine will back up each version of the temporary files in that folder. It is best to add these folders to Time Machine's "Privacy" list.
If you are uncertain which folders these are, you can use a utility like WhatSize or GrandPerspective to visualize the size of folders on your boot drive and on the Time Machine disk, to figure out which files are continuously being backed up.
Beyond refining what Time Machine backs up, try manually invoking the backups when you know you have not modified any documents. This should create a minimal backup of system log files and other small configuration files, but not take a long time to complete. If it does, then something may be wrong with how the drive is flagging changed files.
Do you have any third-party software installed? One that some people have found that uses a lot of the hard drive is Adobe's "Version Cue" software for its Creative Suite package. The software is supposed to run in the background to manage files in various projects, but has been known to use a fair amount of system resources when doing so.
If you have Version Cue installed, go to the system preferences and turn it off unless you use it. Do the same for other software packages and system add-ons, especially those that are used for file management.
Diminished system resources?
The last common reason why hard drives will continually be accessed is if you either are low on hard-drive space, or low on RAM. The boot volume should be kept approximately 10 percent free so the system can adequately manage RAM and virtual memory. If you find yourself using up most of your hard drive, then try moving various libraries (i.e., iTunes, Aperture, and iPhoto) to another drive.
If you are running low on RAM, the system will page out unused RAM to the hard drive to free up more for use by other applications. This is constantly happening on all systems, regardless of the amount of RAM being used, but becomes more important as RAM is being used up. You can check the level of RAM usage by clicking the "Memory" tab in the Activity Monitor utility, and seeing the green value on the pie chart. This value indicates the level of available RAM, and if it is below about one eighth of the chart on average, I would recommend installing more RAM.
Keep in mind that the performance of virtual memory is also affected by how full the hard drive is, so if you are both low on RAM and have little hard drive space left, you will see the performance hits amplify each other. This is why we recommend to keep around 10 percent of the drive free.
Hard drive problems
If there are problems with the volume's structure, you can try fixing them with a drive utility. Apple's "Disk Utility" should be the first option to try, but there are also others, such as DiskWarrior, Drive Genius, and TechTool Pro, that can help diagnose problems with the drive and volume structure. Run these utilities on all drives, including Time Machine (keep in mind the many hard links on the Time Machine drive may result in it taking a very long time to be checked by these utilities).