There are three ways that I know of to transfer files to a B2 from a computer
1. Via a USB drive , the music files and the artwork files need to be in folders with the hierarchy Artist\Album\Tracks on the USB drive
2. Use the Web UI file upload functionality
3. Connect your computer to the B2 HDD as network attached storage (NAS) and just copy the files using your computer's file manager application. It very important to run a 'Scan Disk' after copying with this method because you are circumventing the B2 software and the B2 needs to rebuild its index of your music files.
Options 2 & 3 assume your B2 and computer are on the same home network
Its recommended you do the transfer in batches and whichever way you do it its going to take a fair amount of time
There is a lot of information on the Brennan website about loading files to the B2
John