Hello, this is how it works for me.
There are two ways.
1. Turn samba on. Check on file explorer under Network, you should see Android something visible.
sda (\\ANDROID_AA7DD\share\media_rw) is how you should see it.
If you have both internal and external disks you should see two.
sda1 (\\ANDROID_AA7DD\share\media_rw)
sdb1 (\\ANDROID_AA7DD\share\media_rw)
2. Install from playstore FTP File Transfer app. Start it, and the free app should open an ftp port, you see the exact address under settings. Also there you configure the shared folder.
THEN, you go to laptop and open FILEZILLA (fastest), configure it to open the above address, and there you go, FASTEST file tranfer to both internal or USB HDDs.
Cheers.