To store data from your project onto an external SD card when using an Android device, just open up the Fieldwire app and click the three vertical dots in the top right corner. Select the "Data" option and you'll then see the option to switch between storage "disks." Disk 1 is the your mobile device's main storage space, Disk 2 will be your SD card. Even if you have an SD card inserted into your Android device, you will still need to manually switch to external storage in the Fieldwire app settings before it begins storing data there.
Switching to SD card storage will help prevent using up all of the device's storage space. Plans, files, photos, and videos can all be stored on the SD card, and you can switch between storing on Disk 1 or Disk 2 at any time in that "Data" menu.
Please keep in mind that when you switch storage location, all of the data on the original storage disk will be deleted and re-download onto the new storage disk (in other words, it won't remain on both). Please also note that if you are not connected to WiFi at the time of switching storage options (thus causing the re-download of data to the other disk to be handled by your mobile device's built-in internet service), this may eat up some of your data plan. So be sure you're connected to WiFi to avoid that.