Backup to disk then to tape

For Symantec to backup to disk and then to tape, must the disk be an external disk storage or can the server internal hard disk be used for that purpose?

The statement “A BasicDisk type storage unit consists of a directory on locally-attached disk or network-attached disk….” , can someone clarify the meaning of locally-attached disk? Does it mean an internal hard disk or an external hard disk attached directly to the server?

Thank you.