I have just finished developing a rails app which has a mysql db as a backend.
The app is meant for high traffic and will store lots of information. I am planning to set up my own web server and host the site from it.

If in future my disk space runs out i would want to expand by adding more space.
But say if my mysql database is housed in my /disk0s1 and by adding a new drive i have more partitions (and hence more disk space), how then would i extend my database to store information on those partitions too, and at the same time prevent any information from being written on the original partition. Should i go for multiple databases? if so how?

If i went for a hosting solution i wouldn't be bothering about this as i would just have to worry about making payments for the extra space :)
I always wondered how space is added on-the-go by these webhosts.
Is there any specific mysql configuration that i have to make?