I'm not sure, but I don't think it is possible to install XAMPP on another disk. XAMPP needs to be installed on the startup disk. I believe many of the XAMPP paths are 'hard-coded'. So, if you move XAMPP to another folder it will complain it can't find file '/Applications/XAMPP/xamppfiles/...'.
There are other possibilities to travel with your websites, though. You can make use of virtual hosts to point to a folder on an other location of the disk. I've no experience yet with pointing to files on another disk, but in http://community.apachefriends.org/f/viewtopic.php?f=29&t=41004&start=0
it's discussed. The only bad thing is that you can't carry your databases this way, but a solution for that would be to use (or write) a PHP-application that regular makes backups of your databases and stores them together with the webfiles. You could even write/use another PHP-application that automatically will detect new backups made on an other location and restores everything when needed.