Can only see DBR_BOOT and DBR_Master

After switching to a different WD external drive and putting My Book Essential back on, I can only see DBR_BOOT and DBR_Master in the directory.  How do I get my data back?  What happened?

Saw another post on another site that this happened to someone who did a system backup to the external drive.  There weren’t any answers that post.  I did, in fact, do a system backup very recently.  Perhaps that’s what happened but the question still remains, how do I get my data back?