What OS? I've seen this happen several times under 98/SE when doing a large copy operation. I don't think I've ever seen it of 2000 and I imagine XP should be the same. If it happens again, try copying fewer folders/files at a time. On 98, it seerm to be more related to the number of files than the total size of the copy. You might also try turning off anti-virus--just a wild guess. Worst case, you might try booting into "safe mode" and copying them from there with a minimum of loaded drivers, etc. Again, just a guess.

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