gparted automatically mounts USB and other removal partitions when it does it's initial scan as well as during operations. This is particularly a problem with coping partitions, which is often this case if you want to copy a partition from local disk to USB or between USB partitions. Even if you unmount just before hitting "Apply" gparted will mount all the drives then give an error that partitions must be unmounted before operation can occur. 

Other information:
Work around for Ubuntu 8.10 is to disable automounting all together using this command:

gconftool-2 --type bool --set /apps/nautilus/preferences/media_automount false