My ideas are the following:
1. This has to do with Xubuntu installation, not with Gparted. I don't know details about Xubuntu installation and configuration, but I think that it installs what is needed for the computer that installer sees at the install time. If you move the drive to another computer, the configutation isn't automatically changed to the new hardware configuration. To prepare a disk with system installed for another computer, is a rather advanced option and you need to work manually on the system config files.
Furthermore, drives A:, B: C: etc are named that way by windows, not by Linux systems like Ubuntu. It seems that the disk contains still a windows boot loader. Normally, you must install the Linux boot loader (Grub, Lilo, ...) in the master boot sector.
You have better to ask details about all this installation procedure in Ubuntu forums or general Linux forums, like LinuxQuestions.org.
2. Gparted contained in Ubuntu and Xubuntu must be a very old and buggy version of the program, so it could cause errors. Please, use any late version from the Gparted main site
http://gparted.sf.net/
But, in your case, the problem must come from the installation procedure, not from Gparted.
*** It is highly recommended to backup any important files before doing resize/move operations. ***