files on filesystems are subject to certain constrains. So can files on Fat32 not exceed 4GB in size. copying a file (20Gb in my case) from in my case ntfs to Fat32 will result in the file copying and getting stuck at 4GB no warning about anything it will just wait for ever and basically do nothing because that constraint can not possibly change. therefor: first check if the source file is bigger than the biggest possible file in the targetfilesystem and then second start copying or throw an error...
Thanks for reporting ! Though there is already an open bug for that problem. Patches would be very welcome ! *** This bug has been marked as a duplicate of bug 13751 ***