By default, updatedb neglects partitions mounted in /media. However, it could be really useful if locate worked on those partitions as well. Especially, if you mount the partitions of your HDDs there (that is a common practice nowadays). If you want to change this behaviour, just remove /media from PRUNEPATHS settings in /etc/updatedb.conf (as a root of course). Note, however, that it is much better to mount the partitions of your HDDs into /mnt and use /media just for removable devices.