backup hung on C:\

Hi folks

2 server window 2003 client server having same problem. NBU version 7.1.0.2. whenever backup runs it stays hung for C: and D: but finishs for shadow copy components.actions taken : killed bpbkar process, CPU utilization is ok, restarted NBU services on client box, completed bpbkar NULL test successfully,checked VSS writers all stable, checkd disk space on C: , D: , eventlog have no errors related to NBU , volsnap or VSS. enabled bpbkar and bpfis logs

please provide any idea on this problem.