There shouldn't be anything in /mnt, unless you have other partitions and had them mount to a directory in the /mnt folder. If the disks are all good (and I imagine a store-bought CD would be) then it's either a bad drive or something very strange in Xubuntu. Have you tried the drive in other machines? Well, I guess it would work if you installed with it... Hm...
Actually, thinking about it a bit, you may need to add it to the fstab. It might not be mounted at all, in which case it won't do anything. Can you do "mousepad /etc/fstab" and post what it says in there? That might be the problem.
And yes, you can rep people at any time, they just don't count until you have 10 posts or more. For a full explanation, there is a thread in the TBCS Handbook forum. I think you -rep'ed me though LOL