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Noted from Apple's Forums: Strange Problem when Copying files to NAS
This can also occur from the default finder window if you have your finder window open as gallery (has large preview as default view) or columns.
Solution
First switch finder in the top right menu bar to "list" so you do not have a preview by default, this may be enough to fix the issue.
It is recommended that on the properties of both the folder you are copying a file from and the folder you are copying to on the Mac to have 'Show Icons' unchecked.
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