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9. Disk Utility Doesn’t Erase Free Disk Space

One of the new features of Tiger is the ability to securely empty the Trash, which is useful when you want to remove all traces of the files therein instead of just deleting the operating system’s references to same. But what about files you’ve already trashed?

Disk Utility’s Erase pane has an Erase Free Space button that gives you the option of writing over a volume’s empty space 1, 7, or 35 times. However, even when you are just zeroing out the free space, for some reason Disk Utility needs to create a temporary file. Judging by my personal experience and many posters on Apple’s own support forums, this time-consuming often results in an alert complaining that the disk is dangerously full (see Figure 9).

Figure 9

Figure 9 Paradoxically, erasing free disk space fills the volume.

Why erasing free space requires creating a huge temporary file is beyond me, especially because doing so appears to prevent the task from completing properly. If you stop the erasure, you need to restart the Mac to reclaim the space stolen by this huge invisible temp file.

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