You can plug in an external USB drive and copy everything to that drive, that's actually a pretty good plan.
If you do so via samba (ie. via your computer over the network) it'll go pretty slow, since it's routed through your computer (at least, I think it is).
The better way to do it is via SSH, using the command cp
(copy). This will be limited to the speed of the USB connection.
Another way to do it, is to take out the hard drive, connect it to your pc using SATA, connect the target (backup) drive the same way and booting into linux (you can use a live USB stick, you won't need to install linux). You can then copy using whatever linux filemanager is in the specific distro. This will be the fastest way in the sense that it's limited by the sata connection, which is faster than usb. It does, however, require some work in taking the hard drive out of the NAS and putting it in your pc.
There's one more thing: if you use one of these three options you can save all the files (data) that are on your NAS. There is also the option of creating an exact duplicate of the drive, in that case if the original HDD breaks you can insert the backup and the nas will continue working without issue. You'll have to connect both drives to a computer and use the dd
command. If that's something you're interested in you'll have to look in to it in more detail.