I am a little disappointed about the lack of response to be honest although I have managed to work it out.
First of all it depends on the type of NAS which you are using as from past experience you will screw it up if you make a mistake. I am using a ReadyNAs and the first thing you need to do is to enable SSH access. Netgear do realise the need that some users have to be able to go in at root access and accordingly they have a file which you can easily update using the update menu on their system. They do however make it quiet clear that in certain cases you may end up voiding the warranty if you go into areas of the root where you shouldnt be going.
So assuming the update has gone okay you now have obtained root access. Now all you need to do is to connect your PC to the nas. Again assuming you are using Windows, search for 'Putty SSH' and download the small software package onto your desktop. Double click the application, insert the IP of your NAS, select SSH and then click on open. A black screen will open and ask you for your login. In my case the login is
ROOT and not
ADMIN, the password is the normal password you use to access the NAS.
Next, connect the USB drive to the NAS and this is where Back Up Assist will tell you, you need to run the following command:
i. Run the following command in your shell: bash "/mnt/usbdrive/SeedFolder/seed.sh"
I am afraid that it isnt as easy as this as you need to actually find where your USB drive is located. The other problem I experienced was that the seed.sh file was not located beneath the SeedFolder as BAS state but in the root of the USB drive.
So to find where your USB folder is located and what it is called you will need to type into your shell the command:
ls /
This will display all folders on your NAS. Once you have found your USB, you need to run the bash command. So in my case the command will be:
bash "/USB_HDD_1/seed.sh"The data should then start to be transferred from your USB drive to the NAS. In my case it did upto a point and then the data stopped transferring.
So although I can now know how to get to that point, I am still struggling transferring the whole of the USB

Hope this helps.