NVDB backups complete with warnings - "Backup xxx is incomplete. Required old streams are missing (4045270)

NVDB backups complete successfully but return warnings.
The log may show the following messages:

Bad segment found
with context:
Segment: Segment is on media that should have been retired: Id 532449
Stream d30a02acb002764f8b7200002c10000001000000
From 850234376192 To 1067814944767
Job 7124 Instance 10 Phase 1
IP address 0xac020ad3 and 0xd30a02ac
Date 21:00 23 Mar 2012 and 07:04 21 Jun 2004

Warning Media Manager Segment Check: 17498 segments examined, 340 segments with no media, 0 orphaned segments, 0 discarded segments

Information 2012/06/26 11:17:37 91 Media NVSERVER Session with incomplete new streams: Backup 156794 Job 6940 Instance 28 Phase 1
Warning 2012/06/26 11:17:38 91 Media NVSERVER Backup 155702 is incomplete. Required old streams are missing
Information 2012/06/26 11:17:45 91 Media NVSERVER No Session matching Backup Index 292196