Dell Powervault 114X - USB - 500GB - Write Protect On

We have a Dell Powervault 114X that is hooked up to a Windows Server 2012 system via a single USB Cable.  The Powervault 114x has only a single RD1000 drive installed in it using TJKJC 500GB Cartridges.  Windows Server sees it as an external USB drive just fine.  It has been working for about 1.5 years.  We use BackupExec 15 to backup some data to it.  We have 4 external cartridges that we swap out for backup purposes.  As of last week, I'm unable to write any data to the cartridge.  I cannot "drag-and-drop" files from Windows nor use Backup Exec to write data to it.  I can read data just fine.  The Dell RD1000 Windows utility shows that the drive is "good", but shows the cartridge as Write Protect: ON.  There is a physical tab on the cartridges that allow write protection, however they are all in the unlocked position.  It is happening on all of the cartridges we have.  I have rebooted the Powervault 114X and the server without any change.  Any ideas what might be causing this?  Thank you.