The subject line is self explanatory.
Try copying from a source that has a deeply nested set of folders -- Toucan fails to copy (and produces an error message for) files that have a total pathname of more than about 250 characters.
Try something like:
C:\Files\Fourscore\and\seven\years\ago\our\fourfathers\ [...] \not\perish\from\the\earth.txt
Maximum allowable pathname size on NTFS filesystems (the most common Windows filesystem) is 32767 unicode characters, with the max length of any single folder or filename of 255. I have some that are about 350-ish in length.
For specs on the maximum pathname for various filesystems, see: