Recreate external data groups on a test LTM to test another LTMs

You can actually SCP the external data group files off the production system. The files themselves are stored in a BIG-IP filestore in a location (path) that uses the following naming convention:

/config/filestore/files_d/_d/data_group_d/

For example, if the external data group is defined in the Common partition, it can be found in /config/filestore/files_d/Common_d/data_group_d. Just list the directory and you should see all the external data groups defined there. The naming convention for a data group is "::__version" - where "" is a randomly generated number and "version" is the version number of the data group (only changes if you replace it with a new version). For example, I created an external data group named "test" in the Common partition, and it's full path and name is as follows:

/config/filestore/files_d/Common_d/data_group_d/:Common:test_62208_1

From there, it's a simple matter to just download it off the prod system and then import it to the test system at System > File Management > Import.