All the NT644 sold pre to November 2014 are supplied with 4GB SD card. Since Foxwell keep issuing new updates to extend vehicle and system coverage, the 4GB SD card would be full if you update all the software to the latest version.
Our suggestion is to replace it with a 8GB SD card. Alternatively, you may delete some of the vehicle software that you are not in need to make room. Please remember it's always suggested to delete and udpate software via the update tool Foxscanner.
To avoid this problem, all the NT644 produced from December 2014 onwards are supplied with a 8 GB SD card .