Backup NAS to an external drive on a connected computer I found some options to backup the Synology NAS to an external drive, that is directly connected to the NAS. But is it possible to backup to a computer's external harddisk? For example, userX starts up his PC and windows connects automatically his network drive to the NAS, it uses the credentials of userX to connect to the NAS. The NAS knows that userX is connected and could start a backup of the files, to the users external drive. In windows I can also set a fixed drive letter, so maybe the NAS can check if the drive is connected?

The things work on different direction. When you map NAS drive to computer, NAS have no access to this computer hardware. So NAS can't backup to computer disk. You can do it with software which run on the computer.