It wouldn't be the FTP client if you're getting disk error messages. Its most likely the server space issues. We need to do some end of summer cleaning up =P
You guys if you need to upload single files you can use the file manager in cpanel. Just to browse to the directory where you want to upload your files and click upload files. However, if you have to upload directories and maintaining same structure, you have to wait. It still works but takes a long time.