From what I understand, on the 61 key, a 2.5" SATA Hard Disk is optional. [standard on the 76]

They both use USB for file storage and transfer. 2 USB Host Front/Rear (2.0 Hi Speed) and 1 USB Device Rear (1.1 Full Speed) connections.

The Complete Specs KORG Pa3X Professional 61 & 76

The 61 key is about $3,700
The 76 key is about $4,000