The controller could be RAID controller or simple storage switches.

So he takes two 120-gig drives, and they're merged by his RAID controller into a virtual 240-gig array.

I can't justify spending $500-800 on a RAID controller, not for a home server.

An example would be a RAID controller that performs no scheduling on its own.

The RAID controller determines which drive gets which chunk of data.

The idea is you've got a controller, the so-called RAID controller, which will read from both drives.

The RAID controller simply asks for those sectors it could not get from the first drive.

The disk formats on different RAID controllers are not necessarily compatible.

With the popularity of motherboard integrated RAID controllers, this is extremely difficult.

Often these additional connectors were implemented by inexpensive RAID controllers.