My experience is that tape drive errors are common with many backup applications. DPM is no difference here. According to posts I found most issues are caused due to incorrect behavior either by tape drive firmware or device driver.
But I found also some issues where DPM fails to span to multiple tapes. This could be solved by creating a registry key: [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft Data Protection Manager\Agent] "BufferQueueSize"=dword:00000001" This will ensure that backup will span to next available whenever any IO device error beyond of 30 GB on tape occurs. There by avoiding backup failures