The Smartbox/Media Streamer uses an industry standard known as "USB Mass Storage Device Class" to connect to external USB storage devices like USB Flash Drives (aka thumbdrive, jumpdrive, pendrive). Due to storage and other limitations on a device like the Smartbox/Media Streamer, the full Mass Storage Device standard is not implemented. So, depending on how your external storage device has implemented that standard, the ultimate limit may be multiple terabyte drives.
Officially, we do not support external storage devices other than the common USB Flash Drive.
We have tested Flash drives as large as 128GB and customers have reported successes with much larger drives. One critical limitation is the power budget available on the USB connector of the Smartbox/Media Streamer. Because the entire system and the battery are so small, it is simply not possible to provide the kind of power that is commonly available on the USB ports of desktop and laptop computers. If you attempt to connect a large external hard disk drive, it must have its own power source.