MBL 3TB, Norton Ghost 15 and Sleep

I’m on the latest MBL firmware and have eventually traced a recurring error to the NAS.  I use Ghost to create off-site copies of my backups on a regular basis, one from a wired PC the other wireless.  The wired PC backs up with no issues at all, whereas the wireless backups always fail after about 10 minutes (about the 6Gb point of the file copy).  The Ghost error is always the same, unable to connect to …(file path to MBL).  Just tried disabling the sleep function on MBL the drive copy worked first (and second) time.  Ticked the sleep box for MBL in the UI and tried again - it failed once more at the 9/10 minute point.  Looks like I’ll have to leave MBL on all the time to get this working.  Strange that the wired connection works fine - it takes up to 30 minutes to complete the file transfer), just the wireless copy does not seem to keep MBL awake.

Router is a Draytek 2710Vn, all PCs are running Win 7.

Anyone got an idea how to fix this?