Backup failed

I was running full multiple database backup. For some database it created the backup file properly & for some it cant. In the Redgate log file contains the folloing mesage:

8/25/2009 10:00:02 PM: BACKUP DATABASE [AG_IW] TO VIRTUAL_DEVICE = 'SQLBACKUP_FF2D8D54-5AF5-48FE-9C6A-C8A606DD29C0' WITH BUFFERCOUNT = 6, BLOCKSIZE = 65536, MAXTRANSFERSIZE = 1048576, NAME = N'Database (AG_IW), 8/25/2009 10:00:02 PM', DESCRIPTION = N'Backup on 8/25/2009 10:00:02 PM Server: D111SQL01\D111SQLI02 Database: AG_IW', FORMAT

8/25/2009 10:00:02 PM: Thread 0 error:
Error 620: Error writing to backup file(s).
Process terminated unexpectedly. Error code: -2139684860
8/25/2009 10:00:02 PM: Warning 210: Thread 0 warning:
Warning 210: Error writing to backup file: \\st.com\it$\sqlbackups_qadev\D111SQL01_D111SQLI02\AG_IW\AG_IW_FULL_20090825_220002.sqb
Warning: System error 1453 (Insufficient quota to complete the requested service)
8/25/2009 10:00:02 PM:
SQL error 3013: SQL error 3013: BACKUP DATABASE is terminating abnormally.
SQL error 3271: SQL error 3271: A nonrecoverable I/O error occurred on file "SQLBACKUP_FF2D8D54-5AF5-48FE-9C6A-C8A606DD29C0:" 995(The I/O operation has been aborted because of either a thread exit or an application request.).
SQL error 3202: SQL error 3202: Write on "SQLBACKUP_FF2D8D54-5AF5-48FE-9C6A-C8A606DD29C0" failed: 1117(The request could not be performed because of an I/O device error.)

Can somebody explain why is it happening?