Hi Colin,
Here are a couple of figures with regard to transfer times to and from a USB device.
The JB7 will transfer data to the USB device at a rate of 1.5GB per hour so if your music collection is 200GB (as an example) then your JB7 will back up to USB in 133 hours or about 5.5 days.
The B2 will restore this 200GB collection at a rate of about 25GB per hour which is about 8 hours.
I have run many transfers to and from JB7s and B2s, so I can say that the transfer process is very reliable. The transfer from the JB7 is very slow so it is useful to prepare for that.
Regards,
Peter.