First of all why are you mapping 192.168.100.1 ? Is that your router's IP address or that was the IP in a guide you found on the internet ?
Second you first must allow your user access to the drive ; Management>User click on the Edit option
and in the Samba/FTP sections choose Read/Write access type and for the "Folder" option choose Browse and go to the root of the drive ( it will list the path in the associated field ) .
With the correct rights assigned and the correct path listed you should be able to map your network drive ( or FTP site if that's your preferred option )
https://imgur.com/a/2JAUQUC
You shouldn't however expect wonders from the SMB share capabilities of the router ; most owners get only ~7 MB/s write speed to the USB HDD (read speeds are however better) and only sometimes ( non reproductible conditions , random ) roughly 20-27 MB/s write speeds .