Block Bitmap Corrupted when trying to clone 1tb disk to 640g disk

I'm trying to clone a 1tb drive to a 640g drive. I've not had problems with this before, but this time I get error message:
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Failed to prepare operations. Error code: 10 'File system error is found'
with extended code: 458,774 'Block bitmap corrupted'
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1) Is this referring to a problem with the source 1tb disk or the 640g destination disk?

2) What can be done to fix this. I don't think it's related to disk size, as I've done this before. and I have plenty of room on the smaller 640 for all files on the source disk.

Thanks!