Таки нарвались
support.microsoft.com/en-us/kb/954037
Error message when you try to copy a file to a DFS shared folder or to a shared folder that is a mount point in Windows Vista or in Windows Server 2008: "The selected files could not be copied. There is not enough free space on the device"
CAUSE
This problem occurs when the volume that contains the DFS root or the mount point shared folder has little disk space. In this situation, Windows Explorer does not calculate the free space of the DFS shared folder or the target volume of the mount point. Instead, Windows Explorer incorrectly calculates the free space of the DFS root or the volume that contains the mount point.
В точности наш случай. Оверрайд - целпять пользователю диск в обход DFS, по прямой шаре. Ну или перемещать корень DFS на другой раздел, побольше. Кстати, не знаю, что там с best practices по поводу размещения этого ресурса.
А теперь - самая мякотка:
Applies to
Windows Vista Enterprise 64-bit Edition
Windows Vista Ultimate 64-bit Edition
Windows Vista Business
Windows Vista Business 64-bit Edition
Windows Vista Enterprise
Windows Vista Ultimate
Windows Server 2008 Datacenter without Hyper-V
Windows Server 2008 Enterprise without Hyper-V
Windows Server 2008 for Itanium-Based Systems
Windows Server 2008 Standard without Hyper-V
Windows Server 2008 Datacenter
Windows Server 2008 Enterprise
Windows Server 2008 Standard
Список не полон: Windows Server 2008 R2 также с этим багом.