Hi, I have a local server with 725GB of storage made up of a number of drives using LVM.
What I did was install your OS (in my case Centos) on one disk then format the other (data) drive as LVM and mount it some where sensible for your setup (/home/user/data ?)
You can the then increase the storage by putting in another physical drive to the system, adding it to the LVM group, and finally extending the LVM partition to take advantage of the new storage capacity.
There's a Nice LVM tool for gnome under centos that helps you do this
Be advised though, LVM is NOT RAID so if one of your HDDs dies then you loose that data (and maybe your whole LVM disk ?)
As a a side note I had to put lots of additional fans in the system to cool it down as lots of HDDs create lots of heat!
Not sure how you specify which disk to use first, maybe someone else can help you with that - would you really notice the speed drop ?