Symptom - While Attaching a case in FTK 3.1.2, you get the error ""No Valid Case Exists at Chosen Location"

Reason -
There are two requirements to be able to Attach a case to FTK 3.1.2:

1. The case must have been originally created/processed in FTK 3.1.x.  This is because the same database schema is shared by these versions, while the schema differs from previous versions.

2. The case must have been archived first.  You can check if a database has been archived by navigating to the case's folder and seeing if there is a folder named something like "DB f-0".  This folder contains the needed database information to restore the case.

Resolution -
Ensure that the case originated from the correct version of FTK.  To archive a case (in the version of FTK that lists the case), open FTK, highlight the desired case, then click Tools>Backup>Archive or Archive and Detach.  After doing this, the cases should Attach fine (Tools>Restore>Attach).

This solution is valid for other versions of FTK also,