[GLLUG] Seemless addition of a hard drive?

[impact]

A quick question.  I run a server which I use to backup all of my 
personal data (music, documents, site-backups, etc), but the hard drive 
I use is getting very close to full.  I have an extra 80gb sitting here 
and would like to add it, but the problem is how to mount it with the 
least difficulty.  All the files are in /var/ftp (FTP server), and It 
would be best If I could somehow mount two drivers (the old one, and the 
new one), to the same place, so I wouldn't have to mount to a folder 
within it.  I understand I can alway just make a /var/ftp/more folder, 
and mount in there, but then I have to keep track of which one has how 
much space, and manually put things on the right drives.

Is there any way to tell linux (running Gentoo 2.6.10), "Heres an extra 
hard drive, whenever something doesn't fit in /var/ftp on the normal HD, 
throw it on the new one"?

-Dan Sherman