Error responses

If a command succeeds it returns information such as the name of the current working directory or a list of files, or OK if there is no information to report.  If it fails, you see a detailed error message instead of the typical ERROR response for a failing AT command. The response is a named error code and a textual description of the error.

Note The exact content of error messages may change in the future.  All errors start with a capital E, followed by one or more uppercase letters and digits, a space, and an description of the error.
If writing your own AT command parsing code, you can determine if an FS command response is an error by checking if the first letter of the response is capital E.