The short answer is No, on the default settings avast only scans files that are accessed.
Simply connecting an external hard drive or flash drive shouldn't make avast scan anything, unless it is active. e.g. as mentioned in a flash drive if it has an autorun.inf file which runs certain files when you connect it, then avast would scan those files which it wants to run.
Personally you don't want to mess with those default settings as USB/Flash sticks are a higher risk of infection if you are in the habit of connecting them to others PCs, etc. Especially if those PCs aren't under your control and protection.
But you should have been able to test that simply by monitoring the file system shield stats before connecting said external hard drive or flash drive and after connection.