This may sound crazy, but do you have any pc's that have the NAS mapped as a network drive? Also any Pc's with shortcuts to directories on the NAS? If so, delete them and restart your pc and NAS. Since I did this my problems appear to have gone!
I had the same problem with a ReadyNas Pro - the first attempt at file transfer would cause the NAS to fall over.
What seems to have worked for me is to change how Windows was accessing the device - Windows Explorer was using the device name ('\\READYNAS'). When I changed to the IP address instead (static -'\\192.168.200.xxx\'), it seems to have worked.
