VMware vSphere Client/vCenter Server Version 4.0.0 Build 208111.

We created a bunch of new LUNs, planning to increase an existing Datastore’s capacity.
VMware side, the operation should be a matter of simply firing up vSphere Client, choosing a host, Configuration tab, viewing the Datastore’s properties, then clicking on the Increase button. Except that no Extent device seems to be found. That’s weird because we already did (multiple times) a rescan of each Storage Adapter/HBA. Moreover, selecting “Add Storage” as if we were to create a new Datastore, indeed shows the expected volumes.
The solution turned out to be this one:

• Connect vSphere Client directly to the host (thus logging in as root), and not to the vCenter Server.