In all honesty, I feel this is overkill every time that you plug in a USB as avast will protect against autorun.inf stuff and scans files that are active, e.g. you try to run them from the USB.
In fact running any program from the USB is likely to come in for more scrutiny (simply because it is from a removable source) it is likely to have the behavior shield and autosandbox intervention.
So given the above the routine scanning of the whole USB is much depreciated, not to mention if you plug in a USB (or external drive) which has a lot of data is likely to have you waiting some time before you can use it.
