By factory reset I assume you mean to remove the configuration file.
The easiest/fastest way to do that is to write a zero to the "filesize" property in the configuration file. That will wipe out the configuration file leaving the BBC-SD the same way it left the factory.
Just remember that if you didn't upload the file to your computer first (atomic file read) you will not be able to undo the wipe you will envoke by zeroing out the file. The file will simply cease to exist.
Object Type: File (10)
Object Instance: 0 (BBC-SD Configuration File)
Property: File Size (42)