QNAP has launched the TS-X51A series , featuring USB 3.0 QuickAccess port that can be used to directly access files and data stored on the NAS. In short you can use it as an External USB Hard drive, as a NAS and as a iSCSI-SAN all the same price.
There are 2 models TS-251A and TS-451A in 2 and 4 bays respectively.