run lsusb and note the Bus and Device IDs for the External HDD (in this case)

Bus 001 Device 002: ID 1058:2621 Western Digital Technologies, Inc.

Create an xml file ExHDD.xml as follows :
<hostdev mode='subsystem' type='usb' managed='yes'>
<source>
<address bus='1' device='2'/>
</source>
</hostdev>


Next just run following command to attach the device :

virsh attach-device fs1 /root/ExHDD.xml

where fs1 is the QEMU guest