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This means either the Firewire interface can't handle that much copying, the disk is too full to support that much copying at once, or something is interfering with the copying like Spotlight indexing which can be set to not index on external drives by dragging them into Apple menu -> System Preferences -> Spotlight -> Privacy if using 10.4 or higher. If none of the above apply, your hard drive may have directory damage which my FAQ explains how to repair:
http://www.macmaps.com/directoryfaq.html
or the cable or hard drive itself is damaged, and it is time to replace the hard drive. So hopefully your data is backed up.
