Adding my config updates here for reference:
Inside the <list> element, I added:
<name id="9">BackupToUSB</name>
Then, just under the closing tag of the list element (</list>) - notice the escaped values in the run tag:
<BackupToUSB>
<count>1</count>
<item id="1">
<method>3</method>
<1>0</1>
<2>2</2>
<3>*</3>
<4>*</4>
<5>*</5>
<run>rsync -vuar /mnt/HD/HD_a2/Main /mnt/USB/USB1_c1/ >/var/log/BackupToUSB 2>&1</run>
</item>
</BackupToUSB>
