Hi Stu, hope that you are enjoying the Brennan! Do you have a B2?
If you connect your external HDD to another machine (Windows, Linux Macbook, etc) you can copy across the contents over the network.
You could either use the Brennan's web user interface on that other machine to do an upload, or setting the Brennan as a NAS might be easier.
Check the Brennan website FAQs on how to set up the NAS, then once that is working you can just copy across the lot. Run Scan Disk once the copying is complete.
If you need more details at any stage then just ask. There's plenty of knowledgeable people on here.
Peter