Just to share my experience with this application. If you are migrating Plesk (8.x) to Plesk (8.x) (ie another server running plesk) and you have little space left on the /home (or /) hard disk partition on the source server.
You must also have a partition that has at least enough space to hold the /var/www/vhost content…
Plesk migration manager stores dumped file under /home/migration. To alleviate the risk of filling the / or /home partition do this:
rmdir /home/migration
ln -s {otherpartitiondir} /home/migration
where {otherpartitiondir} is a full path on the partition that has enough space.
Plesk is not really telling you where the stuff is stored (or I did not find it)