Thanks Marc (Mumsoft).

I can see the contents of the mirror on my Linux machine. I followed all of the steps but on step 4 I created a mirror in the WL700ge. In step 5, I needed to mount from /dev/sdb1.

Not sure why it works or why step 1 is needed. I cannot see the drive in Windows, but that's okay. At least if the internal drive fails I can connect the usb drive to the Linux machine and recover the data on the mirror.

Now I can sleep at night.