Thanks for your efforts to help. This is the full log from the Time Machine error log widget:
Starting standard backup
Backing up to: /Volumes/Time Machine Backups/Backups.backupdb
Error parsing SystemMigration.log to determine source volume of system migration.
No pre-backup thinning needed: 80.64 GB requested (including padding), 374.14 GB available
Error: (2) setxattr for key:com.apple.backupd.SnapshotVolumeUUID path:/Volumes/Time Machine Backups/Backups.backupdb/Thomas Heflick’s iMac/2012-08-04-111701.inProgress/EC78037D-6199-46DA-860C-430E2C5B9011/Audio/Vi sual/BFD2 size:37
Stopping backup.
Copied 0 files (15 KB) from volume Audio/Visual/BFD2.
Copy stage failed with error:11
Backup failed with error: 11
Also, based on your suggestion that there is a bad file on the disk, I excluded all five folders on the external drive from the backup, then, one at a time, included them in the backup to see which folder might have a bad file. As long as ANY folder from that drive is included in the backup, it fails. Doesn't seem to be a bad file, not a visible one anyway.