Backup Exec 2012 hangs with USB harddisk and state Active

Problem:

An USB harddisk is getting spontaneously a status "Offline" according to the messages from Backup Exec en Windows application Log.

Within 1 min (!) the USB disk is getting again spontaneously a status “OK” according to the Backup Exec but has in the same time state “Active” and no backups are possible anymore. Backup Exec 2012 hangs and also the administrator is getting no messages about failing of the backup.

Only killing of Backup Exec Engine service which is hanging and stopping/restarting afterwards all rest of the services of Backup Exec can resolve the problem of  hanging of Backup Exec.

How to avoid this problem? It happens very often.