This HowTo is "Work in Progress" and therefore neither complete nor free of errors, both in writing and content.
We are in no way responsible for crashed systems, loss of data or nights spent repairing computers.
 Remember: you should back up your data!

Scenario:
a well working linux server attached to a storage via scsi with, say, 750GB of HD space in a raid 5.
Now we want to add new (and probably bigger) HDDs to the storage, which still has some empty slots.

1. To insert the disks into the storage should be no problem: cradle out - HDD mounted - cradle in.
2. Make the new disks known to the storage controller (most likely this has already taken place in the time you need to start the application)
3. Create a new Logical Disk out of the new physical disks, something like a raid 5 or raid 6
4. Create a new Logical Volume (ore more than one)
5. Initialize the LD/LV
6. Map the new LD/LV to one or more LUN(s)

After the Initialisation is done, the server needs to know that there is a new disk in town.

The easiest  (but not the most elegant) way is to unload and reload the driver/module for the controller card.
On reload the controller scans all available drives/disks and makes them known to the system.

Some distros have tools like "hotadd", scsidev or the like.
If possible, use them because unloading/loading a module for an active card can get you into trouble very fast!