You don’t say what your operating system is ?
What avast version are you using, latest (excluding any beta versions) is 8.0.1489
The Full System scan scans all drives. If you have your external drive attached, then windows sees it as a drive and assigns a drive letter and avast would scan it.
Where are you seeing this option “Scan also mounted removable volumes (not checked off)” as I can’t see it in the default settings for a Full System Scan ?
You can create a custom scan in which you could exclude the external drive letter, though that would always have to retain the same drive letter.