Q. If I use Windows BitLocker Drive Encryption to protect a volume, is the content of any volumes mounted as folders under the protected volume also encrypted?

A. No. When you create a volume in Windows, you can assign it a drive letter, mount the volume as a folder in an existing volume as shown in the following diagram, or do both.

 As the diagram shows, when you view the D drive you see a folder called Virtuals. When you look in Virtuals, you see the image1, image2, and image3 folders as if they were part of the D drive when they are, in fact, stored on a separate volume, linked to from D.
If you encrypt the D drive using BitLocker, you’re performing volume-level encryption. Bitlocker encrypts the entire D volume but any volumes mounted as folders are still completely separate volumes and therefore are not encrypted, as the following diagram shows.