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The problem arises because of a fundamental difference in the way that Mac OS X handles files compared to previous versions of the OS, and only occurs when when the OS is installed on a Mac OS Extended (HFS+) partition - the standard type for most OS X installs.

HFS+ allows files to have two 'forks,' for data and resources, a feature that is usually exploited by Classic and Carbon applications to store code and other resources. But Unix, which lies at the heart of Mac OS X, uses a 'flat' filing system that has only a single stream for each file.

This means that,

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when backing up from the command line using standard Unix tools like tar, resource forks are not copied into the back up file, and no indication is given that the back up has failed. Users could then restore from the back up, only to find that any file using a resource fork is irretrievably corrupted. Applications built using Cocoa, Mac OS X's advanced API, do not use resource forks and so are immune to the problem.

In addition, simply copying files to a second drive using the Mac OS X Finder will fail to backup any invisible files, which are essential for the OS to properly function. Mac back up applications run in Classic will not work, as they will not be able to handle Mac OS X links.

This means that, at present, there is no reliable back up method for Mac OS X. Although a beta version of Retrospect Client is available from Dantz's Web site, this software is currently unsupported. Furthermore, it must be used in conjunction with Retrospect running on a Mac OS 8 or 9 server.

Read the full report in the forthcoming issue of MacUser magazine.