Currently Being ModeratedOct 31, 2012 7:58 AM (in response to LuisBorges)
How is your external drive formatted? This problem usually occurs, if you are copying from/to a drive that is not formatted Mac OS X Extended, and usually, if your Photo Library contains files with filename that include national characters, like äöüé, etc.
If your drive is still the default MS-DOS (FAT) reformat it using Disk Utility, and then try again to copy.
If this does nor help, look at the part of your Photo Library, that has been copied. You can open the library by ctrl-clicking it in the Finder, then select "Show Package Contents". From the files that have been copied, try to figure out, which file would have been copied next and causes the error. Check, if you can open it in Preview or another program. Look, if the filename contains non-standard characters.