Comment 6 for bug 86851

External drive or not, this bug seems to be a duplicate of bug #37768. I started GParted from the Feisty live CD, tried to resize an NTFS partition on the built-in SCSI harddisk. This failed at resizing the actual file system (that is, after resizing the partition), obviously because the partition was automatically mounted after the partition has been resized. The problem did not occur with the following workaround:
https://bugs.launchpad.net/ubuntu/+source/gparted/+bug/37768/comments/25