Hi,
When a connection fails, active FTP or SFTP sessions represented by Ftp or Sftp objects fail as well. If you need to resume a transfer, you have to do that explicitly. For single-file transfers, you might use this approach: https://www.rebex.net/sftp.net/features/single-file-operations.aspx#resume
For multi-file transfers (Upload and Download methods), specify ActionOnExistingFiles.ResumeIfPossible option.