A simple workaround could be to use the functions available in the operating system to combine two or more physical disk volumes to a big logical volume. In this way you can build almost as large disks as you want. No need to split your calibre library any more, and not even any need to buy new hard drives, if the old ones are healthy and big enough when combined.
This is done using a Logical Volume Manager (LVM) and/or some form of raid or striping for fault tolerance and better performance.
http://en.wikipedia.org/wiki/Logical_volume_management
http://en.wikipedia.org/wiki/Logical_Disk_Manager