Full Scan ext HD?

Hi Forum

 

The following advice was offered to me by a local electronics store techie when I got my new ext HD. 

     "Run a Full Scan on the ext HD after creating every full image backup and on a schedule because it is easier for my Security product to find hidden / dormant malware on the ext HD then on my HDD"

 

Is there any truth to this.... and why?   

 

Thanks

bjm_