Locate and manage files on extern hard drive through FileZilla over SSH

I have installed Debian Server on one of my servers and plugged in a extern hard drive to it through USB (sdb1, 2, and 5 on the image below) where I have all my files on right now. Before I used this WD Red hard drive (that are in a cabinet right now) in a NAS from Synology and I have not formatted it and I don't want that either. Now I want to manage these files through FileZilla over SSH. I can't boot into Synology's operating system on my server because of unknown reasons. Therefore I installed Debian Server. I know that I can install Xpenology on a USB drive and boot it from there but I don't know how so Debian is the first choice at this moment. FreeNAS is no go since it requires 8 GB in RAM. My server only have 1 GB or 1,5 GB in RAM. How can I access my files through FileZilla over SSH? Where can I find the location of the files on the extern hard drive and so on? Is mount the correct command for this? How can I use it?