Thanks for your feedback and sorry for the inconvenience this have caused.
There are many models of Android devices, and due to the fragmentation of Android, non-standard design of SD card path, AirDroid might not display the capacity of the SD card correctly.
Our team are working on this and will try to fix this in future updates.
Here's a temporary solution: you may try to access your external SDcard on the web desktop by the path like "System" > "/mnt/extSdCard" in the "Files" module.