Error: "The operating system returned the error '32(error not found)' " on running backup with "F (4351839)

From SQL ERRORLOG

- Error: 3633, Severity: 16, State: 1.
- The operating system returned the error '32(The process cannot access the file because it is being used by another process.)' while attempting 'CreateFile' on
'D:\Microsoft SQL Server\MSSQL.1\MSSQL\FTData\TestDB_Search\SQL.HDR' at 'fulltext.cpp'(529).
- Backup Error: 3041, Severity: 16, State: 1.
- Backup BACKUP failed to complete the command BACKUP DATABASE ARIES WITH DIFFERENTIAL. Check the backup application log for detailed messages.

According to books online:
During a full database backup in SQL Server 2005, full-text data is backed up together with other database data.
During the backup, the catalog is put into a read-only mode, so that "crawl" activity (the process creating and maintaining a full-text index) is suspended until the backup completes.