odd behaviour with network CopyTo function

Hi all.
anyone come across this before?
running SQL Backup 7.2.1.4 on a Windows 2008 R2 server.
doing a differential backup, every hour.
part of the process copies the backups to a network share, on a Buffalo Terastation also running W2008R2.
at 0900 (and earlier) it works, saying:
03/01/2013 09:00:01: Copied H:\SQLBackup\Differential\DIFF_(local)_model_20130103_090000.sqb to \\crdbackup2\crdecms3\differential\DIFF_(local)_model_20130103_090000.sqb

at 1000, it gives a warning saying it can't create a folder:
Warning 141: COPYTO error: Unable to copy H:\SQLBackup\Differential\DIFF_(local)_model_20130103_100000.sqb to \\crdbackup2\crdecms3\differential\DIFF_(local)_model_20130103_100000.sqb ().
03/01/2013 10:05:31: COPYTO error: Failed to create folder: \\crdbackup2\crdecms3\differential
there is a quota on the share folder, of 100Gb, of which 2.18Gb is used.

i'm stumped.

John