How to change the filesystem protocol, SLDB protocol is not allowing to append a file?

If you are using the File Writer Snap, The Append File action is supported for the file, FTP, FTPS and SFTP protocols only. To write to one of these locations, you change the File name field to point to the appropriate protocol path, such as sftp://ftp.snaplogic.com:22/dir/filename, and create an account for that. See https://docs-snaplogic.atlassian.net/wiki/spaces/SD/pages/1438927/File+Writer