Howtogeek posted a tutorial about repairing the missing external hard drive database error in Windows Home Server

If you're using external hard drives with your Windows Home Server, they might get unplugged and create an error. Here we look at running the Repair Wizard to quickly fix the issue.
If an external drive that is included in your drive pool becomes unplugged or loses power, you might see the following error under Home Network Health when opening the WHS Console. To fix the issue verify the drive has power and is plugged in correctly and click Repair.

  Repair Missing External Hard Drive Database Error in WHS